Doc: Make explicit that description is in RST format

This avoids a 'twine' warning.

Author: omarkohl

setup.py

@@ -33,6 +33,7 @@ def _read(fname):
     maintainer_email='omarkohl@gmail.com',
     description=DESCRIPTION,
     long_description=LONG_DESCRIPTION,
+    long_description_content_type='text/x-rst',
     entry_points={
         'pytest11': ['pytest_datafiles = pytest_datafiles'],
 	},