MAINT: apply reST file extension for Github recognition.

Author: rkern

README.txt renamed to README.rst

@@ -12,4 +12,4 @@ doctest-tests=1
 universal = 1
 
 [metadata]
-description-file = README.txt
+description-file = README.rst

setup.cfg

@@ -6,7 +6,7 @@
 
 # Read the long description from the README.txt
 thisdir = os.path.abspath(os.path.dirname(__file__))
-with open(os.path.join(thisdir, 'README.txt')) as f:
+with open(os.path.join(thisdir, 'README.rst')) as f:
     kwds['long_description'] = f.read()