Update README.md (#218)

README.md

@@ -21,8 +21,10 @@ We recommend users start with our [tutorial folder](https://github.com/som-shahl
 ```bash
 pip install femr
 
-# If you are using deep learning, also run...
+# If you are using deep learning, you also need to install xformers
+# Note that xformers has some issues with MacOS. You might also need to install llvm. See https://stackoverflow.com/questions/60005176/how-to-deal-with-clang-error-unsupported-option-fopenmp-on-travis
 pip install xformers
+
 ```
 # Getting Started