Change README.md to README.rst

PyPi renders text in reStructuredText so it would be better to
have the README in reStructuredText than Markdown.

README.md

-# synapse-diaspora-auth
-
-Diaspora authentication for Matrix Synapse.
\ No newline at end of file

README.rst

+synapse-diaspora-auth
+=====================
+
+A diaspora authenticator for matrix synapse.
\ No newline at end of file