Write `CONTRIBUTING.md`

[jswrenn]

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.

[Philippe-Cholet]

I recently contributed about each of those points (except DoubleEndedIterator) and there are key points I would have liked to know early on so I think I can help here. I'm starting a draft: Philippe-Cholet/itertools@master...contributing
Mentorship welcomed.

EDIT: Markdown rendered here. I try to be as complete and precise but brief as possible.

[Philippe-Cholet]

• I would like some feedback on my draft of "CONTRIBUTING.md".
• However, even if I am familiar with all kind of tests we do here, I did not write anything about testing yet as it's still unclear to me what should be suggested exactly. I think some clarification would be useful.
• After seeing "specialization tests" a few days ago, I had ideas to extend this further. I'd like to know if such thing would be of interest (we would discuss this further in another issue):

tests/specializations.rs definitely seems promising (I really like the Unspecialized struct) but all apply some "closure" only on a fresh iterator and not on an iterator that has yielded elements. A buggy iterator could behave differently if those were applied later on.
I've experimented quite a bit (and eventually ditched macros, for sanity) to check size_hint/count/last/nth/fold/collect (as most of our specialized methods are those) at every step of iteration (at the start and after every next). And it works fine but is slower as you might expect, so I added the possibility to limit this to a few steps.
Usage (example) would remain nice but my implementation is definitely way more verbose (working branch: master...Philippe-Cholet:itertools:test-specializations).

[jswrenn]

Can you file a draft PR for CONTRIBUTING.md?

I'm very interested in seeing our specialization tests improved further!