Fix 'ref' format errors in README file

While releasing new version in pypi, it perform the
validation on the package and give error if there is any
formatting issue.

README file is being used to upload on pypi and so validation
check fails[1] on current README due to 'ref' used for internal
doc link. Those links would not be available for outside the built doc.
This can be checked locally by:
    python setup.py check -s -r

This commit fixes those format issue by removing the ref and provide the
external links.

[1] http://logs.openstack.org/17/622317/1/check/test-release-openstack/a24116d/job-output.txt.gz#_2018-12-05_06_16_03_329613

Change-Id: I70c995b715184c2cc70b494c185c83b8190629db

README.rst

@@ -1,7 +1,8 @@
 Introduction
 ============
 
-hacking is a set of flake8 plugins that test and enforce the :ref:`StyleGuide`.
+hacking is a set of flake8 plugins that test and enforce the
+`OpenStack StyleGuide <https://docs.openstack.org/hacking/latest/user/hacking.html#styleguide>`_
 
 Hacking pins its dependencies, as a new release of some dependency can break
 hacking based gating jobs. This is because new versions of dependencies can
@@ -76,7 +77,8 @@ Requirements
 ------------
 - The check must already have community support. We do not want to dictate
   style, only enforce it.
-- The canonical source of the OpenStack Style Guidelines is :ref:`StyleGuide`,
+- The canonical source of the OpenStack Style Guidelines is
+  `StyleGuide <https://docs.openstack.org/hacking/latest/user/hacking.html#styleguide>`_,
   and hacking just enforces
   them; so when adding a new check, it must be in ``HACKING.rst``
 - False negatives are ok, but false positives are not