Add contributions guidelines

[dzhlobo]

Clarify who does version bump. Should maintainer only do this or not?

[tavaresb]

I think this should be done by the maintainer(s) only. I only did a version bump in my fork because i'm using this in production (and i needed my bundler to fetch a new version of this gem).
I completely forgot to start a feature branch so that i could make a pull request only with the code changes, my bad.