Add doc for adding custom ops (#2509)
Summary:
Add documentation for adding custom ops.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/2509
Reviewed By: xuzhao9
Differential Revision: D64497281
Pulled By: FindHao
fbshipit-source-id: 20f4096ebbce53c7d9a713cacbde016c521aa7c3
