Create README for distributed benchmark (#1183)
Summary:
Add readme so distributed benchmark is easy to run and understand.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1183
Reviewed By: xuzhao9
Differential Revision: D39559898
Pulled By: erichan1
fbshipit-source-id: 2a77a72e3f03dd5acb2ad388412a0a1ef65e0a64
