[Contributing] update contribution guide for 2.7/3.0

[xabbuh]

Q	A
Doc fix?	yes
New docs?	no
Applies to	all
Fixed tickets	#4508

[wouterj]

Like it 👍

[stof]

👍

[javiereguiluz]

Probably I'm very late to this discussion, but here it is my comment.

I like what @xabbuh has done and everything is correct. But what do you think about transforming that long paragraph into a more actionable list-based format. Something like this:

Before working on a patch, you must determine on which branch you need to work:

* ``2.3``, if you are fixing a bug for an existing feature;
* ``2.7``, if you are adding a new feature which is backward compatible;
* ``master``, if you are adding a new and backward incompatible feature.

[xabbuh]

@javiereguiluz NIce suggestion. What do the others think?

[wouterj]

@javiereguiluz it's never too late :)

I think it's a great suggestion which would make things a lot easier to understand.

[xabbuh]

Updated the pull request.

[xabbuh]

Anything else you see that should be changed?

[weaverryan]

This reads super well - great work guys :)