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

Change-Id: I54489b2beda898103e622316d3095b54393039d6
Reviewed-on: https://go-review.googlesource.com/5414
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

npat-efault/sys,darkliquid/golang-sys,kim-racktop/sys,kim-racktop/sys,npat-efault/sys,golang/sys,stevenh/golang-sys,golang/sys,darkliquid/golang-sys,kim-racktop/sys,npat-efault/sys,darkliquid/golang-sys,EricLagergren/sys-1,zhaoming200808/sys,multiplay/golang-sys,npat-efault/sys,stevenh/golang-sys,multiplay/golang-sys,multiplay/golang-sys,zhaoming200808/sys,golang/sys,darkliquid/golang-sys,zhaoming200808/sys,stevenh/golang-sys,EricLagergren/sys-1,EricLagergren/sys-1,multiplay/golang-sys,EricLagergren/sys-1,kim-racktop/sys,stevenh/golang-sys,zhaoming200808/sys