Update OSS build instructions for submodule build (pytorch#110)

Summary:
Pull Request resolved: pytorch#110

Due to the update the ASMJIT submodule in the near future, we add the additional instruction for submodule update for OSS users.

Reviewed By: dskhudia

Differential Revision: D16415043

fbshipit-source-id: 0488dda34a5916a40eee948a5b7455cf8770d72d

README.md

@@ -64,6 +64,9 @@ General build instructions are as follows:
 ```
 git clone --recursive https://github.com/pytorch/FBGEMM.git
 cd FBGEMM
+# if you are updating an existing checkout
+git submodule sync
+git submodule update --init --recursive
 mkdir build && cd build
 cmake ..
 make