Break up CONTRIBUTING.md a bit; add section on pull request labels
Break up CONTRIBUTING.md a bit; add section on pull request labels

mozilla/kuma,escattone/kuma,ollie314/kuma,jwhitlock/kuma,jwhitlock/kuma,jwhitlock/kuma,jgmize/kuma,mozilla/kuma,Elchi3/kuma,SphinxKnight/kuma,yfdyh000/kuma,Elchi3/kuma,jgmize/kuma,jwhitlock/kuma,Elchi3/kuma,ollie314/kuma,a2sheppy/kuma,yfdyh000/kuma,safwanrahman/kuma,escattone/kuma,ollie314/kuma,yfdyh000/kuma,a2sheppy/kuma,safwanrahman/kuma,jgmize/kuma,a2sheppy/kuma,mozilla/kuma,SphinxKnight/kuma,Elchi3/kuma,SphinxKnight/kuma,mozilla/kuma,SphinxKnight/kuma,escattone/kuma,ollie314/kuma,jwhitlock/kuma,a2sheppy/kuma,jgmize/kuma,SphinxKnight/kuma,safwanrahman/kuma,jgmize/kuma,yfdyh000/kuma,a2sheppy/kuma,yfdyh000/kuma,ollie314/kuma,SphinxKnight/kuma,ollie314/kuma,yfdyh000/kuma,mozilla/kuma,jgmize/kuma,safwanrahman/kuma,Elchi3/kuma,safwanrahman/kuma,safwanrahman/kuma