document the concept of a standard external_url field for gatewayed events

drafts/as-http-api.rst

@@ -425,6 +425,15 @@ SHOULD be mapped in the same way as "user" URIs.
     alias: <complete room alias on local HS>
   }
   
+Event fields
+~~~~~~~~~~~~
+We recommend that any gatewayed events should include an `external_url` field in
+their content to provide a way for Matrix clients to link into the 'native'
+client from which the event originated.  For instance, this could contain the
+message-ID for emails/nntp posts, or a link to a blog comment when gatewaying
+blog comment traffic in & out of matrix
+
+  
 Examples
 --------
 .. NOTE::