Update README.md (#20088)
Summary:
Sometimes people need to checkout an older version and build PyTorch. In that case, they need to do `git submodule sync` and maybe `git submodule update --init` as mentioned [here](https://github.com/pytorch/pytorch/issues/20074).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20088
Differential Revision: D15195729
Pulled By: soumith
fbshipit-source-id: 73232b801e5524cdba462dd504fb973d95d0498c
