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

Change-Id: I9ed184d0eac9f0435aa6dbe9c8743a7bdd9a498f
Reviewed-on: https://go-review.googlesource.com/5212
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

tamnd/tour,LaPingvino/go-tour-eo,rofrol/go-tour-pl,raindev/tour,rofrol/go-tour-pl,LaPingvino/go-tour-eo,Jasperste/tour,phoorichet/tour,LaPingvino/go-tour-nl,dupoxy/go-tour-fr,campoy/go-tour-ua,dupoxy/go-tour-fr,Go-zh/tour,LaPingvino/go-tour-nl,dupoxy/go-tour-fr,golians/tour,scryner/tour,bbandaru/tour,golians/tour,bbandaru/tour,bbandaru/tour,tamnd/tour,LaPingvino/go-tour-nl,campoy/go-tour-ua,campoy/go-tour-ua,shuLhan/go-tour-id,raindev/tour,tamnd/tour,Jasperste/tour,golians/tour,Go-zh/tour,Jasperste/tour,LaPingvino/go-tour-eo,shuLhan/tour,ybin/tour,ybin/tour,shuLhan/tour,shuLhan/tour,zorion/go-tour-ca,scryner/tour,zorion/go-tour-ca,raindev/tour,Go-zh/tour,scryner/tour,golang/tour,phoorichet/tour,ybin/tour,Go-zh/tour,rofrol/go-tour-pl,scryner/tour,shuLhan/go-tour-id,shuLhan/go-tour-id,phoorichet/tour