Summary:
Add a small top-level build.sh script that invokes
build/fbcode_builder/getdeps.py to perform the build.  This primarily helps
make the getdeps build process more discoverable to users.

Also document this script in README.md

Reviewed By: yfeldblum

Differential Revision: D23484822

fbshipit-source-id: 6013ace6d80d7d3d80aa87d4c6269ac106814cf0