Switch to extensionless README and LICENSE files

Since the README and LICENSE files are going to get included into
reStructuredText files for documentation rendering, they don't
really need .rst extensions and this keeps them more consistent with
the AUTHORS and ChangeLog files generated by PBR.

diff --git a/LICENSE.rst b/LICENSE
similarity index 100%
rename from LICENSE.rst
rename to LICENSE

diff --git a/README.rst b/README
similarity index 100%
rename from README.rst
rename to README