Add ninja to PyTorch README.md file. (#20079)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/17572
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20079
Differential Revision: D15195388
Pulled By: zhangguanheng66
fbshipit-source-id: b7448b482f07e96753f727664416a5d0e85602b4
