Update the description content type to markdown

[wbingli]

Issue #, if available:

Description of changes: Need the right description type to publish.

Testing

Before

 twine upload --repository-url https://test.pypi.org/legacy/ dist/* Uploading distributions to https://test.pypi.org/legacy/ Enter your username: aws-cloudformation-developers Enter your password: Uploading cloudformation_cli_python_plugin-2.0.0-py3-none-any.whl 100%|█████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 23.3k/23.3k [00:01<00:00, 21.5kB/s]NOTE: Try --verbose to see response content. HTTPError: 400 Client Error: The description failed to render in the default format of reStructuredText. See https://test.pypi.org/help/#description-content-type for more information. for url: https://test.pypi.org/legacy/ 

After

 twine upload --repository-url https://test.pypi.org/legacy/ dist/* Uploading distributions to https://test.pypi.org/legacy/ Enter your username: aws-cloudformation-developers Enter your password: Uploading cloudformation_cli_python_plugin-2.0.0-py3-none-any.whl 100%|█████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 23.3k/23.3k [00:02<00:00, 11.9kB/s]Uploading cloudformation-cli-python-plugin-2.0.0.tar.gz 100%|█████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 21.5k/21.5k [00:01<00:00, 20.2kB/s] View at: https://test.pypi.org/project/cloudformation-cli-python-plugin/2.0.0/ 

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[johnttompkins]

In the future let's add this into the CI to verify the pypi packages can package: https://github.com/aws-cloudformation/cloudformation-cli/blob/master/.travis.yml#L18