Update the description content type to markdown (aws-cloudformation#105)

Author: Wenbing Li

setup.py

@@ -27,6 +27,7 @@ def find_version(*file_paths):
     version=find_version("python", "rpdk", "python", "__init__.py"),
     description=__doc__,
     long_description=read("README.md"),
+    long_description_content_type="text/markdown",
     author="Amazon Web Services",
     author_email="[email protected]",
     url="https://github.com/aws-cloudformation/aws-cloudformation-rpdk-python-plugin/",