Summary:
Update the `README.md` file in the fbcode_builder subdirectory to document the
current getdeps.py build system rather than the older fbcode_builder scripts
that required each project to keep their own python-based build files in their
project directory.

Reviewed By: chadaustin, wez

Differential Revision: D23591356

fbshipit-source-id: 75a099a10793e68a2b59696682010c4dff47ec69