DOCS: Add a CONTRIBUTING.md

After looking through a Github [cheat sheet](https://github.com/tiimgreen/github-cheat-sheet), i found that Github allows for [contribution guidelines](https://github.com/tiimgreen/github-cheat-sheet#contributing-guidelines) that shows up when trying to open a Pull Request. (An example is show in the cheat sheet).

I thought it would be helpful to add such a file, to make the relevant information a bit more accessible, and indeed visible for new contributors. I thus created a draft of such a file, making sure to drag out what I remembered as important, but also to supply links to all the relevant wiki pages. Overall this should provide an easier starting point for new contributors ("Where do I find the information you're complaining about me not having read?" should be answered).

This should also add a solution to "Nobody told me about any commit guidelines/code formatting convention/portability rules!".

Overall, this is just a draft, and very open to discussion, correction, fixing etc. Markdown was chosen as Github automatically formats such documents.

Thoughts?