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

Change-Id: I927934bc58d956d711f78656ed851c02c16ec490
Reviewed-on: https://go-review.googlesource.com/5216
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/playground,adams-sarah/playground,adams-sarah/playground,heshed/playground,phoorichet/playground,heshed/playground,neollie/playground,gophergala2016/playground-plus,phoorichet/playground,golang/playground,neollie/playground,adams-sarah/playground,neollie/playground,phoorichet/playground,heshed/playground,golang/playground,gophergala2016/playground-plus,gophergala2016/playground-plus