README.md: link to go2 template
README.md: link to go2 template

Change-Id: I9100ea7b6221206e8a40717086334d91f87a617e
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/309209
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal