Create a contributions guide

[mpizenberg]

There has been many more active contributions in the past few months and I realize we don't have any guideline except the small paragraph in the readme. I think it would be great to add one, so that new contributors feel less lost in the code base. Here are the few bits I remember we either discussed explicitly, or I tried to hold implicitly.

• Github default branch is release which stays in sync with the latest published version to avoid confusion by reading stuff that diverged from the published package
• The branch for current work is dev, this is the one people should PR against
• Of course, make sure code is formatted, tests pass, clippy is happy
• Avoid cyclic module imports
• Avoid pub(crate) in public modules. If we need it others might need it. Or find a better way.
• Private modules live in the internal namespace
• Avoid code in modules that are just "folders"/namespace. Stick to just doc comments of the namespace.

There might have been more. And of course, since I'm less active its fine if these guidelines change. In any case, it would be useful to have something.

[zanieb]

Some notes on pull request title standards would be good too.