Improves CONTRIBUTING.md
Improves CONTRIBUTING.md

openstreetmap/iD,kartta-labs/iD,AndreasHae/iD,digidem/iD,AndreasHae/iD,kartta-labs/iD,openstreetmap/iD,morray/iD,openstreetmap/iD,morray/iD,digidem/iD,kartta-labs/iD,AndreasHae/iD