Polish contributing guidelines

List of TODOs:

• s/Contribute/Contributing
• Add matrix where IRC is mentioned
• Make a clear "Submit merge requests" section that also contains information about how to work with gitlab
• One change per commit is a bit too narrow, also fine to get a well-focused merge request
• Add that public types, functions, and constants should be documented. (XXX: Would be good to cross-check in CI.)
• Add that name should be entered in authors file (related to #4 (closed))
• Move "maintainers" here, since this is the place where it's relevant