Provide a migration strategy from python-gflags to absl-py

[gpshead]

https://pypi.python.org/pypi/absl-py/ (https://github.com/abseil/abseil-py) replaces this library. We need to document the migration strategy.

Also consider if there is even more we could do such as having python-gflags depend on absl-py or even push a version implemented on top of it? Though it'd be a lot better to not provide such a crutch and just reach out to anything with a python-gflags dep to have them update to absl-py directly.