Update README.md

Add comment to README about installing cryptography when using PyPy

Author: Mat001

README.md

@@ -32,6 +32,12 @@ To install:
 
     pip install optimizely-sdk
 
+Note:  
+If you are running the SDK with PyPy or PyPy3, install this cryptography package first and then optimizely-sdk package:  
+
+    pip install "cryptography>=1.3.4,<=3.1.1"
+
+
 ### Feature Management Access
 
 To access the Feature Management configuration in the Optimizely