Update README (bazel-contrib#237)

Expand the Overview. Add more context on core rules vs packaging rules and
level of support. Add instructions for using Bazel Federation. Move pip stuff
to sub-headings.

Pass --isolated to pip (bazel-contrib#232)

This fixes an issue where if you have bad URLs in your ~/.pip/pip.conf,
pip would pick them up and end up failing bazel. This way we make
pip_import more hermetic and ignore user config.