Add userbenchmark doc in readme (#1506)
Summary:
Update readme for new users to quickly find out about the userbenchmark.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1506
Reviewed By: aaronenyeshi
Differential Revision: D44462085
Pulled By: xuzhao9
fbshipit-source-id: 5e40fff7763307c5432bc045b383867e41b98fed
