Use README.rst instead of .txt in MANIFEST.
Use README.rst instead of .txt in MANIFEST.

Currently getting a warning: no files found matching 'README.txt' when installing.
django-cumulus/django-cumulus,elsmorian/django-cumulus,bennylope/django-cumulus,ImaginaryLandscape/django-cumulus,elsmorian/django-cumulus,SmithsonianEnterprises/django-cumulus,SmithsonianEnterprises/django-cumulus,absoludity/django-cumulus,mandx/django-cumulus,rizumu/django-cumulus,ferrix/django-cumulus,bennylope/django-cumulus,absoludity/django-cumulus,CoachLogix/django-cumulus