docs: add limitations, contributing provider, more screen shots

docs/index.md

@@ -1,12 +1,39 @@
 # Electronic Payments Documentation
 
-The Electronic Payments application extends ERPNext[^1] with the capability to receive electronic payments via several vendors directly in the system. The current supported vendors are [Authorize.net](www.authorize.net) and [Stripe](stripe.com).
+The Electronic Payments application extends ERPNext[^1] with the capability to send and receive electronic payments via several vendors directly in the system. The current supported vendors are [Authorize.net](www.authorize.net) and, with some limitations (see Provider Limitations section for details), [Stripe](stripe.com).
 
-Electronic payments is set up to allow portal users to log in and add credit card (Stripe and Authorize.net) and ACH (Authorize.net only) payment methods associated with their account. Note that the payment method details are not saved on your system at any point. The app immediately passes the data to the provider API and, if the API successfully creates a payment method, it only saves the provider's token and last few account or card digits to identify that payment method. The Electronic Payments tab on the Customer page for that party shows a table with any stored payment methods.
+## Installation, Configuration, Settings, and Permissions
+
+There are a few prerequisite steps to get the Electronic Payments application up and running on your ERPNext site. See the following pages for details on installation, configuration, settings, and permissions:
+
+- [Installation Guide](../README.md)
+- [Configuration and Settings](./configuration.md)
+- [Default Permissions and Accounting Workflows](./permissions.md)
+- Refer to the [Example Data page](./exampledata.md) for instructions around installing fictitious demo data to experiment with using the Electronic Payments app
+
+## App Feature Overview
+
+Once the app is installed and configured, you can begin utilizing its features.
+
+Electronic Payments is set up to allow portal users to log in and add credit card (Stripe and Authorize.net) and ACH (Authorize.net only) payment methods associated with their account. Note that the payment method details are never saved on your system at any point. The app immediately passes the data to the provider API and, if the API successfully creates a payment method, it only saves the provider's token and last few account or card digits to identify that payment method.
+
+<!-- TODO: add screen shots of portal and how to add a portal payment method -->
+
+The portal also allows the customer to remove or edit a saved payment method. When making changes to an existing method, they must re-enter all information, since it's not stored in ERPNext and providers don't allow requests for payment method details.
 
-<!-- TODO: add screen shot to add a portal payment method -->
+There's an important consideration regarding the Electronic Payment Settings and payment methods being added via the portal. As noted in the [Configuration and Settings page](./configuration.md), Electronic Payment Settings are specified on a per-company basis. When a portal user adds a payment method, there's no way to associate it to a company, so the app uses the *default company* set in ERPNext to find the provider and API keys to use. This will link the payment method to the default company's provider account. For this reason, an Electronic Payments Settings document should always exist for the default company.
 
-When the customer is logged into the portal and has one or more payment methods set up, they can make a payment on an Order or Invoice directly from the portal. The app automatically integrates with the payment schedule defined on the Terms tab of the document, and will display payments spit out by payment term and showing any valid discounted amounts and due dates as necessary. If the provide accepts the payment and it successfully goes through, the app updates the payment schedule.
+In the desk view, stored payment methods are visible on the Electronic Payments tab in that party's page.
+
+![Screen shot showing a new tab on a Customer page for Electronic Payments. There's a table with one payment saved, which has a mode of payment of Credit Card, a label of Card-9711, the Default box checked, and the Subject to Credit Limit and Service Charge boxes unchecked.](./assets/ep_customer_portal_pmt_methods.png)
+
+Some payment method details may be edited from this view - if the party is subject to a credit limit, or if there are service charges that should be added when the party uses that payment method, those are configurable in the table.
+
+![Screen shot showing the edit detail of the payment method table in a Customer's page.](./assets/ep_edit_payment_method.png)
+
+When the customer is logged into the portal and has one or more payment methods set up, they can make a payment on an Order or Invoice directly from the portal. The app automatically integrates with the payment schedule defined on the Terms tab of the document, and will display payments spit out by payment term and showing any valid discounted amounts and due dates as necessary.
+
+If the provider accepts the payment and returns a success message, the app creates a Journal Entry or Payment Entry (depending how the Electronic Settings are configured) and updates the payment schedule.
 
 ![Screen shot of the portal view of an invoice showing two payment terms, one is already paid and the other has a button to make a payment for that term's amount.](./assets/ep_portal_payment_terms.png)
 
@@ -16,13 +43,12 @@ The app also allows a desk user to make an advance payment on a Sales Order or a
 
 ![Screen shot showing the dialog box to make a payment using a saved payment method. The Mode of Payment is Saved Payment Method: Card 0002 and the Card Number is **** **** **** 0002.](./assets/ep_desk_dialog.png)
 
-## Installation, Configuration, Settings, and Permissions
+## Provider Limitations
 
-There is some required prerequisite setup to get the Electronic Payments application up and running on your ERPNext site. See the following pages for details on installation, configuration, settings, and permissions:
+There are some limitations with using Stripe as a provider. First, only credit card payment methods (not ACH ones) are configurable. Stripe uses its own mandate workflow (to verify that the customer allows making a charge to their bank account) that is currently not supported by the app. Second, given that sending payments to suppliers is only possible via an ACH payment method, that feature isn't available for Stripe.
 
-- [Installation Guide](../README.md)
-- [Configuration and Settings](./configuration.md)
-- [Default Permissions and Workflows](./permissions.md)
-- Refer to the [Example Data page](./exampledata.md) for instructions around installing fictitious demo data to experiment with using the Electronic Payments app
+## Adding a Provider
+
+The Electronic Payments app maintainers welcome contributions to expand provider options beyond Stripe and Authorize.net. To be considered, a provider should have a comparable feature set to those of Authorize.net, including receiving credit card payments and sending and receiving ACH payments. Feature requests and pull requests can be made on the [app's GitHub repository](https://github.com/agritheory/electronic_payments).
 
 [^1]: [ERPNext](https://erpnext.com/) is an open-sourced Enterprise Resource Planning (ERP) software that provides a wide range of business management functionality. Its core features include support for accounting, inventory, manufacturing, customer relationship management (CRM), distribution, and retail.