Add a file containing Contributing Guidelines

[myrdd]

I suggest to add a CONTRIBUTING.md file. Copied from #484 (comment):

CONTRIBUTING.md should contain info on high priority bugs, how to report issues, debug, and donate to each individual contributor.

I've stumbled upon this „template“: https://github.com/contribute-md/contribute-md-template/blob/master/contribute.md (website: http://contribute.md/)

Sidenote: As soon as such a file is created, a link to that file is displayed when a contributor creates an Issue or opens a Pull Request [1].

[1] https://github.com/blog/1184-contributing-guidelines

[nodiscc]

This issue is being worked on at #578