[release-branch.go1.8] doc: reorganize the contribution guidelines into a guide

Updates #17802

Change-Id: I65ea0f4cde973604c04051e7eb25d12e4facecd3
Reviewed-on: https://go-review.googlesource.com/36626Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-on: https://go-review.googlesource.com/38312