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

Change-Id: Ica8168a510824e9519902a3b660025bcf913f762
Reviewed-on: https://go-review.googlesource.com/5413
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

golang/image