Tell PyPI long description is markdown (#101)

This will make it render properly there, as per this SO post: https://stackoverflow.com/a/26737258

Author: adamchainz

setup.py

@@ -19,6 +19,7 @@
     description='The AWS X-Ray SDK for Python (the SDK) enables Python developers to record'
                 ' and emit information from within their applications to the AWS X-Ray service.',
     long_description=long_description,
+    long_description_content_type='text/markdown',
 
     url='https://github.com/aws/aws-xray-sdk-python',