add support component

docs/rfcs/header-component-api.md

@@ -354,8 +354,9 @@ A special header layout for editorial pages. The logo and supporter messages are
 #### Example
 
 ```jsx
-<EditorialHeader isSignedIn showMessage>
+<EditorialHeader>
     <Masthead>
+        <EditorialHeader.Support />
         <Masthead.Links>
             <Masthead.Link href icon={<Icon />} label until from />
             <Masthead.Menu icon label until from>
@@ -377,11 +378,23 @@ A special header layout for editorial pages. The logo and supporter messages are
 
 #### Props
 
--   `isSignedIn` _boolean_
+-   `edition` _string_ `'uk'` | `'au'` | `'us'` | `'international'`
+
+#### `<EditorialHeader.Support />`
+
+Presents either the Support buttons, or the Thank You message depending on whether the user is a supporter
+
+![Support buttons](images/header-component-api/support-buttons.png)
+![Support thank you message](images/header-component-api/support-thank-you.png)
+
+#### Props
+
+-   `showSupportButtons` _boolean_
     -   whether to show the support buttons
 -   `showSubscriberMessage` _boolean_
     -   whether to show the Thank You message
--   `edition` _string_ `'uk'` | `'au'` | `'us'` | `'international'`
+-   `readerRevenueLinks` _{ subscribe: string; support: string; contribute: string }_
+    -   urls to assign to the support buttons
 
 #### `<EditorialHeader.EditionSwitch />`