Rename README.md to README.rst

The content looks more like restructured text to me,
and it seems to be more common amongst OpenStack projects.

Change-Id: Iba8c21985c055f1712069a6e70d345bb920381bf

MANIFEST.in

@@ -1,6 +1,6 @@
 include AUTHORS
 include LICENSE
-include README.md
+include README.rst
 include ChangeLog
 include tox.ini
 include .testr.conf

setup.cfg

@@ -2,7 +2,7 @@
 name = python-heatclient
 summary = OpenStack Orchestration API Client Library
 description-file =
-    README.md
+    README.rst
 author = OpenStack
 author-email = openstack-dev@lists.openstack.org
 home-page = http://www.openstack.org/