Update README by adding release notes URL

Currently, README.rst does not contain release notes URL. So this
patch aims to add a URL that links to release notes of Tacker. It
also creates a bulleted list for External resources session.

Change-Id: I666c013b465edb650558d5a55e505b4a3dba3566

README.rst

@@ -42,8 +42,11 @@ Please report bugs at: https://bugs.launchpad.net/tacker
 External Resources:
 ===================
 
-Tacker Wiki:
-https://wiki.openstack.org/wiki/Tacker
+* Tacker Wiki: https://wiki.openstack.org/wiki/Tacker
 
-For help on usage and hacking of Tacker, please send mail to
-<mailto:openstack-dev@lists.openstack.org> with [Tacker] tag.
+* For help on usage and hacking of Tacker, please send an email to
+  OpenStack-dev Mailing List <mailto:openstack-dev@lists.openstack.org> with
+  **[openstack-dev] [Tacker]** tag.
+
+* To get release notes: `Tacker Release Notes
+  <https://docs.openstack.org/releasenotes/tacker/>`_