This change updates the README.md to describe the order of resolution
between `options['generate_py_protobufs']`, `--protoc`, and the `PROTOC`
env var.
Since the `setup.py` script is not the main way to find docs, this change
deletes the parts of the docstring that are redundant with `README.md`.
This updates README.md to use an example with pip instead of setup.py.
The README.md is then used as the long_description argument to setup(),
so that the doc shows up on pypi.
This extension allows Python code to be generated from setup.py files, so they
are created as part of a normal Python build. The extension uses an already-existing
protoc binary, which can be explicitly specified if needed.
