Add CONTRIBUTING.md

More info on this kind of file and where it will be shown https://github.com/blog/1184-contributing-guidelines

Inspired by many other's CONTRIBUTING.md I've created one that is a mix of guideline and help.
Shortly this is what it says:
- Relevant commits (no merge or temp commits) and good commit messages
- Workflow, never submit PRs from dev-branch, always create your own. Rebase often
- How to Fork, Clone, Push, and handle changes from upstream. How to update an existing PR.

Fixes #372. Instead of having a discussion on the issue first, I thought it would be easier to have something written to discuss, therefore I submitted this PR without having a discussion on the issue first.