Itertools needs a CONTRIBUTING.md. Key highlights should include:

• Picking the right receiver. (see Ownership issues for Itertools methods #710)
• Testing (especially property-based testing).
• How to implement and robustly test size_hint.

A complete guide should also address:

• When to implement DoubleEndedIterator
• When to implement ExactSizeIterator
• When to implement FusedIterator.

(Anything else?)

Happy to provide mentorship on this.