design: tweaks to Code of Conduct text, add reporting form URL
design: tweaks to Code of Conduct text, add reporting form URL

Update golang/go#13073

Change-Id: Ifc64fc929a3d6d33d316d0b52d54b5f7e2dee8a9
Reviewed-on: https://go-review.googlesource.com/17166
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/proposal,golang/proposal