Added Long Description type (#166)

In order to display long description properly at the pypi.org page we
need to specify mime type `text/markdown` for the long desccription if
readme is in markdown.

Current https://pypi.org/project/disco-py/, doesn't looks nice atm.

setup.py

@@ -32,6 +32,7 @@ setup(
     license='MIT',
     description='A Python library for Discord',
     long_description=readme,
+    long_description_content_type="text/markdown",
     include_package_data=True,
     install_requires=requirements,
     extras_require=extras_require,