Add long_description_content_type for Markdown (GeoNode#11831)

Author: giohappy

setup.py

@@ -28,6 +28,7 @@
 setup(
     version=__import__("geonode").get_version(),
     long_description=open("README.md").read(),
+    long_description_content_type="text/markdown",
     package_data={
         "": ["*.*"],  # noqa
         "": ["static/*.*"],  # noqa