Contributing.md and code style

[Jean85]

As discussed in #41 we are ready to review this. We now have:

• a CONTRIBUTING.md file with a basic guide
• a pre-commit hook that checks code for PSR-2 compliancy
• all the code is now PSR-2 compliant

[ranpafin]

I think only the changelog update remains, then we can merge and close #44

[Jean85]

I think I'll update the changelog after the merge, since it's already started with some unreleased entries on master but it doesn't on this branch.

Good work! :+1: