fix: Add content type to package long description

This will allow the package to be uploaded to pypi, as it was failing
checks.

setup.py

@@ -119,6 +119,7 @@ def read(fname, encoding='ascii'):
         author="errbot.io",
         author_email="[email protected]",
         description="Errbot is a chatbot designed to be simple to extend with plugins written in Python.",
+        long_description_content_type="text/markdown",
         long_description=''.join([read('README.rst'), '\n\n', changes]),
         license="GPL",
         keywords="xmpp irc slack hipchat gitter tox chatbot bot plugin chatops",