Define contribution guidelines

[mariospr]

We should perhaps write a Wiki page with some guidelines to contribute to this project such as the ones they have for WebKit [1] or Chromium [2], even though we might probably start a bit ligther.

In particular we should at least define how the process of landing code should look like such as selecting or creating an issue, providing changes in a PR following a certain convention, how the review process would look like, how code should be landed (e.g. no PR without an issue associated, having either PRs or commits should include a link to the issue...), what the desired coding style is...

Once these guidelines are in place it should be easier both for new people to provide contributions as for current developers to abide by the same rules (and/or evolve them if needed at the same time).

[1] https://webkit.org/contributing-code/
[2] https://chromium.googlesource.com/chromium/src/+/main/docs/contributing.md

[mariospr]

This one is related: #110