Clarify relationship of this repo to the other pypa repos

[mattip]

I would like to modernize NumPy and SciPy's wheel building/upload scripts in the spirit of current PyPA best practices. We have a dependence on an external C package, use cython internally for our code, and (in the case of NumPy at least) are used as a submodule when building a compatible-but-different wheel by intel. I thought to start with this repo as a base and extend it, but then discovered the other repos under the PyPA which seem to do much of the same thing. Before I start with NumPy, it would be nice to understand the relationships between all of the repos.

In addition to this repo, there is the
pypa/sampleproject
pypa/integration-test
pypa/pip-test-package
pypya/docker-python

It would be nice to consolidate what can be joined together and to have each of these describe its relationship to the others. I could issue PRs to do this but do not really understand which are still considered "best practices".

As long as I am writing about the pypa repo landscape, it seems these repos should be made read-only if not deprecated
pypa/forklift
pypa/pep470
pypa/ssl
pypa/get-virtualenv
pypa/setuptools-issue1494