Add usage section to readme.

components/x-article-save-button/readme.md

@@ -20,6 +20,24 @@ Feature                     | Type    | Required | Default value      | Descript
 `csrfToken`                 | String  | no       | none               | CSRF token (will be included in a hidden form field).
 `trackableId`               | String  | no       | `'save-for-later'` | Identifier for tracking purposes
 
+## Usage
+
+The components provided by this module are all functions that expect a map of [teaser properties](#properties). They can be used with vanilla JavaScript or JSX (If you are not familiar check out [WTF is JSX][jsx-wtf] first). For example if you were writing your application using React you could use the component like this:
+
+```jsx
+import React from 'react';
+import { ArticleSaveButton } from '@financial-times/x-article-save-button';
+
+// A == B == C
+const a = ArticleSaveButton(props);
+const b = <ArticleSaveButton {...props} />;
+const c = React.createElement(ArticleSaveButton, props);
+```
+
+All `x-` components are designed to be compatible with a variety of runtimes, not just React. Check out the [`x-engine`][engine] documentation for a list of recommended libraries and frameworks.
+
+[jsx-wtf]: https://jasonformat.com/wtf-is-jsx/
+
 ## Client side behaviour
 
 For users with JavaScript enabled, the default form submit action is prevented, and a custom event (named 'x-article-save-button') will be dispatched on the form element.