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
diff --git a/README.md b/README.md
index fdd4846..3b4ee37 100644
--- a/README.md
+++ b/README.md
@@ -169,7 +169,7 @@
 
 Common
 ```
-conda install numpy pyyaml mkl mkl-include setuptools cmake cffi typing
+conda install numpy ninja pyyaml mkl mkl-include setuptools cmake cffi typing
 ```
 
 On Linux