Implement hermetic pip_import

rules_python_external: fill missing docstrings (bazel-contrib#358)

add missing docstrings for rules_python_external

also remove unused attr on pip_repository repo rule

release: 0.0.3

Add information regarding support (bazel-contrib#290)

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.