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

Change-Id: Ib52f590ff2bfd44acfd37eb3bb545180f157a135
Reviewed-on: https://go-review.googlesource.com/5210
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

bford/golang-x-crypto,lucevers/crypto,wooutl/golangcrypto,conseweb/golangcrypto,mattharden/crypto,thomaso-mirodin/crypto,ScriptRock/crypto,conseweb/golangcrypto,cmars/tools,zj8487/crypto,a-palchikov/crypto,kim-racktop/crypto,golang/crypto,ScriptRock/crypto,golang/crypto,cheffo/crypto,cheffo/crypto,cmars/tools,juniorz/crypto,wooutl/golangcrypto,kim-racktop/crypto,Carrotman42/crypto,pki-io/crypto,CodyGuo/crypto,tca/crypto,guaiguaihw/crypto,golang/crypto,pki-io/crypto,lucevers/crypto,cmars/tools,guaiguaihw/crypto,bford/golang-x-crypto,tca/crypto,mattharden/crypto,Carrotman42/crypto,a-palchikov/crypto,juniorz/crypto,thomaso-mirodin/crypto,cmars/tools,zj8487/crypto,CodyGuo/crypto