Introduce CAVEATS section for tricky install situations

Hi!
I propose to introduce a section in README with a collection of possible tricky install situations.

As compilation of HHVM takes a few hours on the sufficiently weak computer, we should warn a user about possible problems which he may face.

I think the first situation should be XQuartz bug which is mentioned in #28 and #11. There are also a lot of people with the same problem in https://github.com/facebook/hhvm/issues/1580.

X11 is commonly used across developers so the repo have to solve the problem completely or warn the user at least.

The unlink and then link procedure seems OK for a quick fix before someone doesn't resolve the issue - sadly, #23 didn't help.

What do you think?