Improve instructions at beginning

Author: david-christiansen

message-index/index.html

@@ -7,10 +7,17 @@ <h2>Welcome</h2>
 <p>
   This site describes the various messages that can be returned by
   GHC, the most-commonly-used Haskell implementation, including both errors and warnings. Beginning with version
-  X.Y.Z, GHC emits a code with the format <code>[GHC-123]</code> for each
+  9.6.1, GHC emits a code with the format <code>[GHC-12345]</code> for each
   message. These codes can be looked up below for further information.
 </p>
 
+<p>
+  Other Haskell-related tools are welcome to add their own messages to the index. Please contact
+  <a href="https://haskell.foundation/contact/" target="_blank" rel="noopener noreferrer">the Haskell Foundation</a>
+  or <a href="https://github.com/haskellfoundation/error-message-index/issues">open an issue</a>
+  to begin the process.
+</p>
+
 <p>
   So far, $messageCount$ errors and warnings are documented here.
   If you encounter a message that is not yet documented, please