Add docs/README.md to make existing doc build info more discoverable (#49286)
Summary:
Closes gh-42003
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49286
Reviewed By: glaringlee
Differential Revision: D25535250
Pulled By: ezyang
fbshipit-source-id: a7790bfe4528fa6a31698126cc687793fdf7ac3f
