Add cffi-specific install instructions and docs

I ran into a really dumb problem when trying to install this - I kept trying to compile lxml using pypy and it kept failing, which led to a lot of frustration. I've added some docs to the page about the cffi project and installation in general to try and avoid having others fall into the same pitfall.

I left a section about production readiness half-empty because I am not sure what steps have been done to test the library and/or who's currently using this in production.