add requirements.txt and fix README.md

[hengruo]

Where is absl-py? And what's msgpack?

[gen-ko]

absl.py is not explicitly required in my test using this requirement.txt setting, install msgpack could silent some warning during the installing of dependencies.

[hengruo]

I'm afraid msgpack can't replace alps-py. alps-py is a part of tensorflow now so you can use absl even if you don't install absl.

[gen-ko]

No, they are not replacing each other. I figured out now, msgpack is required by tensorboard.