Add draft CONTRIBUTING.md file.

[yfarjoun]

Description

Added a CONTRIBUTING.md file to codify standard process of maintaining code.

This is a very generic version, looking forward to a discussion regarding how to make it more specific.

[bwlang]

I think it's very helpful to include this sort of document.

I wonder if you want to add something setting expectations about timing for review
e.g.
"Pull requests will be reviewed on a best effort basis by the core team "...
"We prioritize bug fixes over new features " ...

if you want layered community review / prioritization , maybe set up a mechanism for those?

I've seen other projects write an explicit AI section defining and discouraging sloppy contributions (not sure if that's an issue here).

[tfenne]

I think this is a great starting place.