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

Change-Id: Icd4a3c290b5fa9358042292db4f46a5e151e48c2
Reviewed-on: https://go-review.googlesource.com/5411
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

golang/text,cmars/tools,npchp110/text,cmars/tools,cmars/tools,cmars/tools