Merge: Added contributing guidelines and link from readme

Merge: Added contributing guidelines and link from readme

As discussed in #2133, contribution guidelines are hard to find.

This PR is an attempt to ease the process by giving newcomer friendly advices and some more insight on the contribution process for Nit.

@tonygaetani, do you think a document like this one would have been helpful for you ? Is there anything you would change to make it friendlier/clearer ?

Close #2133

Pull-Request: #2134
Reviewed-by: Jean Privat <jean@pryen.org>