github: add note about using play.golang.org
@@ -11,7 +11,10 @@ of the problem, prefixed by the primary affected package.
### What did you do?
-<!-- Please include a link to a minimal reproducer here. -->
+<!--
+Please include a link to a minimal reproducer here. Preferably this should
+be a complete runnable example at https://play.golang.org/.
+-->
### What did you expect to happen?
