Update Contributing.md

[zoepage]

This is about updating the Contributing.md with the suggested new process for labels and feature requests.

[karlcow]

@zoepage Could you give more details? or links to the discussions/proposals? Thanks. (for the paper trail 😉 )

[zoepage]

https://groups.google.com/forum/#!topic/mozilla.compatibility/Y9fceIecTME
https://groups.google.com/forum/#!topic/mozilla.compatibility/_kCr_EUZDoA