update README for CGAL

[steo85it]

Just as a reminder, the README needs to be updated to allow CGAL compile (and some parts clarified for embree, too).
(I can also do it, not assigning to anyone specific)

[steo85it]

plus, I would replace "==" by ">=" in requirements.txt, unless a specific (potentially old) version is really required (it's currently not the case).

[sampotter]

Yep please go ahead. If you put together a PR I'll be happy to take a look at it.