Update readme with supporting os (#315)

Author: yangaws

README.rst

@@ -53,6 +53,11 @@ You can install from source by cloning this repository and issuing a pip install
     python setup.py sdist
     pip install dist/sagemaker-1.7.1.tar.gz
 
+Supported Operating Systems
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+SageMaker Python SDK supports Unix/Linux and Mac.
+
 Supported Python versions
 ~~~~~~~~~~~~~~~~~~~~~~~~~