Simplify legally required steps in contribution process

[ThomasGreiner]

Background Our current contribution process requires the contributor to fill out and submit to us our Contributor's License Agreement. While we can make this more apparent by introducing a CONTRIBUTING.md file (see #11), we should also make that process simpler and more convenient for contributors.

What to change One way to tackle this would be a more seamless integration with GitHub (see https://github.com/flattr/flattr-extension/pull/9#issuecomment-372440402). Apart from that we could also look into alternatives to CLAs such as a Developer Certificate of Origin (see DCO).