Add link to CSS label from issue tracker to homepage

Author: david-christiansen

message-index/index.html

@@ -24,7 +24,8 @@ <h2>Welcome</h2>
   <a href="https://github.com/haskellfoundation/error-message-index/issues">open an issue</a> or
   <a href="https://github.com/haskellfoundation/error-message-index/pulls">submit a pull request with documentation</a>.
   Pull requests with additional examples and improved explanations for
-  existing messages are also very welcome.
+  existing messages are also very welcome, as are
+  <a href="https://github.com/haskellfoundation/error-message-index/issues?q=is%3Aissue+is%3Aopen+label%3Acss">improvements to the generated site</a>.
 </p>