update first time user guide with authentication detail

docs/gpts/first-time-users.mdx

@@ -2,16 +2,26 @@
 
 After creating your Superface account, you will be directed to your GPT Actions configuration page.
 
-If this is your first time using Superface, you will see the getting started guide.
+## Add a tool
 
 In order to properly connect Superface to your GPT, you will need to have at least one tool installed. To get you moving quickly you can add the [Wttr.in](https://wttr.in) tool directly from this page by clicking **+ Add**.
 
 ![First time users will see this getting started guide](/img/gpts/firstrun-no-tools.png)
 
+## Schema URL & Authentication
+
 Once the tool has been added, the page will refresh with the configuration details you need to connect your OpenAI custom GPT to Superface via an Action.
 
 ![After adding the tool the page will refresh with schema information.](/img/gpts/firstrun-tool-added.png)
 
+## User Authentication
+
+Additionally, you will need to choose the type of authentication you want Superface to use. The default is **Use my credentials**. You can find out more about using **User-provided credentials** in the [User Authentication guide](./user-authentication.mdx).
+
+![User authentication choice](/img/gpts/gpt-user-authentication-choice.png)
+
+## Test your GPT with Superface
+
 At this point you will need to head to the configuration of your custom GPT, and [set up your GPT Action](./setup). Once this is completed you can test your connection with Superface.
 
 ![Waiting for a connection from OpenAI to come through.](/img/gpts/firstrun-try-it.png)