adding AppVeyor badge to readme

Author: mattsb42-aws

README.rst

@@ -13,6 +13,9 @@ aws-encryption-sdk
 .. image:: https://travis-ci.org/awslabs/aws-encryption-sdk-python.svg?branch=master
    :target: https://travis-ci.org/awslabs/aws-encryption-sdk-python
 
+.. image:: https://ci.appveyor.com/api/projects/status/v42snaej4lavd5lm/branch/master?svg=true
+   :target: https://ci.appveyor.com/project/mattsb42-aws/aws-encryption-sdk-python-m2mgl
+
 The AWS Encryption SDK for Python provides a fully compliant, native Python implementation of the `AWS Encryption SDK`_.
 
 The latest full documentation can be found at `Read the Docs`_.