[capi-sig] Include/README

As Victor mentioned earlier today, there is work in progress on documenting the C API hierarchy: https://github.com/python/cpython/pull/24884

I've received valuable reviews from Victor and Guido, but I'd like to have a few more eyes on it.

Most of the text is stolen from bpo-43416, bpo-35134, PEP 384, and PEP 652. I've tried to put it together as concise as possible.

Any kind of feeback is highly appreciated.

Thanks,

Erlend