Add contribution guidelines

[chippmann]

Describe the problem or limitation you are having in your project:
We should define some contribution guidelines to streamline the development process between contributors.
It definitely should include sections about how to branch and write commit messages.

Describe how this feature / enhancement will help you overcome this problem or limitation:
It would make the development more consistent and the commit messages more meaningful. It also would help new contributors to get started more easily.

Show a mock up screenshots/video or a flow diagram explaining how your proposal will work:
None to provide

Describe implementation detail for your proposal (in code), if possible:
As a starting point:
@piiertho and I settled down for git flow as on how to manage branches and write commit messages: https://danielkummer.github.io/git-flow-cheatsheet/ (ignore the plugin, it is not needed)
Rest (and maybe also this one) can be discussed further.

If this enhancement will not be used often, can it be worked around with a few lines of code?:

Is there a reason why this should be in this project and not individually solved?:
As it concerns all contributors the guidelines should be in this project.