README.md in onnxruntime-0.4.0 vs README.md in onnxruntime-0.5.0

@@ -12,9 +12,15 @@
 
 ```ruby
 gem 'onnxruntime'
 ```
 
+On Mac, also install OpenMP:
+
+```sh
+brew install libomp
+```
+
 ## Getting Started
 
 Load a model and make predictions
 
 ```ruby