[quant] Add back README.md for backend_config (#70964)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70964
Accidently deleted before, adding this back. We'll make this more complete after
the structure is finalized
Test Plan:
no test needed
Imported from OSS
Reviewed By: dagitses
Differential Revision: D33470738
fbshipit-source-id: 00459a4b00514d3d0346de68788fab4cad8a5d12
