Add userbenchmark doc (#1328)
Summary:
We are adding the documentations on how to develop a TorchBench userbenchmark for our customers.
Related to https://github.com/pytorch/benchmark/issues/1293
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1328
Reviewed By: davidberard98
Differential Revision: D41778804
Pulled By: xuzhao9
fbshipit-source-id: 89e019ad2014aa0b60b82c7104b93fa63b71e169
