Add a CONTRIBUTING.md file

[stockholmux]

I was just answering a question on the discussions forum about contributing to the docs.

It would probably make sense to have a contributing file in this repo to help people with similar questions about how to get started contributing.

[nastena1606]

Hi! I'd like to work on this one but I have a question, if it's still needed. There is the info in the README. Does it make sense to create a separate Contributing guide for documentation and move the topics from Readme there?