update public README about supported platforms, compilers
update public README about supported platforms, compilers

Summary:
The README stated "folly requires gcc 5.1+ and a version of boost..." which
some people might thing means that folly only supports gcc.

Edit the README with more details about supported compilers and platforms.
I tried to strike a balance between informative (FB uses folly on many
platforms), conservative (the CMake build might not support all of them),
and not mentioning specific versions (likely to get outdated.)

Reviewed By: yfeldblum

Differential Revision: D21093741

fbshipit-source-id: 3a541f22b30736201361d2a10d861d61bd278e47

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly