Improve contribution documentation

[mpoisot]

I ran into a few bumps following the contributing guide. I'll submit a pull request with my ideas.

[jakubpeksa]

There's the newest 4.0 version, are you planning to update the pr?

[mpoisot]

I don't know if the my changes to the docs are accurate anymore, especially since I heard v4 was a pretty big rewrite. Our team isn't planning to update to 4.x for at least a few months.