docs: Clarify documentation location and online access

Updated documentation section to clarify online access and local structure.

Author: fsbraun

README.rst

@@ -57,7 +57,9 @@ To run all the tests the only thing you need to do is run::
 Documentation
 =============
 
-We maintain documentation under the ``docs`` folder using rst format.
+The documentation is online on `readthedocs <https://djangocms-versioning.readthedocs.io>`_.
+
+We maintain documentation in this repository under the ``docs`` folder using rst format.
 
 To generate the HTML documentation you will need to install ``sphinx`` (``pip install sphinx``) and ``graphviz`` (as per your operating system's package management system). You can then generate the docs using the following command:
 
@@ -112,4 +114,4 @@ do not forget to run the ``compilemessages`` management command.
 
 .. |CmsVersion| image:: https://img.shields.io/pypi/frameworkversions/django-cms/djangocms-versioning.svg?style=flat-square
     :target: https://pypi.python.org/pypi/djangocms-versioning
-    :alt: django CMS versions
+    :alt: django CMS versions