[pruner] add README to repo (#64099)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64099
adding readme to pruner in OSS
ghstack-source-id: 136867516
Test Plan: should not affect behavior
Reviewed By: z-a-f
Differential Revision: D30608045
fbshipit-source-id: 3e9899a853395b2e91e8a69a5d2ca5f3c2acc646
