github: direct questions to gonum-dev
<!--
Please make sure your issue title matchs the Go convention; a summary
of the problem, prefixed by the primary affected package.
+
+If the issue is a question about usage or similar, please ask at the
+Gonum discussion list rather than filing an issue:
+
+https://groups.google.com/forum/#!forum/gonum-dev
-->
### What are you trying to do?
