Move (and improve) contributing information from README.md to CONTRIBUTING.md

We have a small section in `README.md` about contributing that should be moved into it's own file. Additionally, the content in that section needs the following improvements:

- Instead of detailing steps (add title, description etc) to open PRs/issues, simply encourage (and thank) people to add issues and open PRs. Then provide a short and practical list of guidelines that might be useful for beginner contributors such as "check existing issues first before opening a new one". The idea is to avoid having arbitrary style guidelines (like having description, steps to reproduce, expected behaviour, alternatives etc) and be more practical.
- Coding standards seems like a useless section. People who know won't need to be told and people who don't know can't write good code just by reading "write good code". I think `CONTRIBUTING.md` should focus only on the git/vcs side of things.