Added RTD badge to README

Author: agronholm

README.rst

@@ -4,6 +4,9 @@
 .. image:: https://coveralls.io/repos/github/asphalt-framework/asphalt-exceptions/badge.svg?branch=master
   :target: https://coveralls.io/github/asphalt-framework/asphalt-exceptions?branch=master
   :alt: Code Coverage
+.. image:: https://readthedocs.org/projects/asphalt-exceptions/badge/?version=latest
+  :target: https://asphalt-exceptions.readthedocs.io/en/latest/?badge=latest
+  :alt: Documentation Status
 
 This Asphalt framework component provides a pluggable means to send exception reports to external
 services. Optionally, it can also install itself as the default handler for exceptions occurring in