doc/gccgo_contribute: mention gofrontend-dev@googlegroups.com mailing list

R=iant
CC=golang-dev
https://golang.org/cl/7323064

doc/gccgo_contribute.html

@@ -100,7 +100,9 @@ or <code>gcc/testsuite/go.dg</code> directories in the GCC repository.
 
 <p>
 Changes to the Go frontend should follow the same process as for the
-main Go repository, only for the <code>gofrontend</code> project
-rather than the <code>go</code> project.  Those changes will then be
-merged into the GCC sources.
+main Go repository, only for the <code>gofrontend</code> project and
+the<code>gofrontend-dev@googlegroups.com</code> mailing list 
+rather than the <code>go</code> project and the
+<code>golang-dev@googlegroups.com</code> mailing list.  Those changes
+will then be merged into the GCC sources.
 </p>