Remove README from docs build

Change [1] introduced a remote image link in our
README.rst. Due to the fact that we include this
README in our docs, our 'tox -e docs' build now
fails due to the nonlocal image URI as we run the
sphinx build with -W:

--
Warning, treated as error:
README.rst:None: WARNING: nonlocal image URI found:
http://governance.openstack.org/badges/neutron-lib.svg
--

Ignoring specific warnings is supported with newer
versions of sphinx [2], but not with the version we
currently use.

Therefore this patch removes our readme reference
from our doc/source/ tree.

[1] https://review.openstack.org/#/c/402702/
[2] https://github.com/sphinx-doc/sphinx/issues/2466

Change-Id: I8458635f7081ea98a160cbb4105032d435bdcb98

doc/source/index.rst

@@ -45,7 +45,6 @@ Programming HowTos and Tutorials
 .. toctree::
     :maxdepth: 3
 
-    readme
     conventions
     installation
     usage

doc/source/readme.rst

@@ -1 +0,0 @@
-.. include:: ../../README.rst