doc: add CONTRIBUTING.md
doc: add CONTRIBUTING.md

Change-Id: I5e6b1e3b308ae7907eacd0a23e2aae6e02a54b57
Reviewed-on: https://go-review.googlesource.com/5215
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

gorakhargosh/blog,gorakhargosh/blog,jeffreyliguori/blog