Add CFFI thread safety docs

[ngoldbaum]

c.f. #187

Adds new docs describing the thread safety guarantees CFFI offers. Includes a worked example demonstrating how to use a thread-unsafe C library safely with a threading.Lock.

[arigo]

Using a global lock like this is necessary it is not safe for more than one thread to simultaneously call into any part of the library. Missing "if" or "when" in that sentence?

As of version 2.0, CFFI generates thread-safe bindings to C libraries. What's new in 2.0 is the GIL-free version of CFFI. CFFI has always generated thread-safe bindings with its GIL-using version, and now in 2.0 it continues to do so in its GIL-free version. I think that most of what you say in your (otherwise great and useful!) text applies equally to both versions.

[ngoldbaum]

You’re right, what’s there right now isn’t correct. I’ll fix it tomorrow. Thanks for giving this a read.

[ngoldbaum]

I’ll fix it tomorrow.

Fixed, I also took the opportunity to expand the text a little with some more advice.

[clin1234]

LGTM

[mattip]

LGTM, will merge in a day or two unless another reviewer has more comments.