fixes rst 'unkown target' error (celery#522)

$ rst2html.py --version; rst2html.py README.rst > /dev/null
....rst2html.py (Docutils 0.12 [release], Python 3.4.3, on linux)
....README.rst:73: (ERROR/3) Unknown target name: "django-celery documentation".

Based on my experience, this should also fix the formatting issue on the
pypi.python.org site

Author: zsoldosp

README.rst

@@ -71,10 +71,11 @@ Documentation
 =============
 
 The `Celery User Manual`_ contains user guides, tutorials and an API
-reference. Also the `django-celery documentation`_, contains information
-about the Django integration.
+reference. It also has a dedicated `subsection about the Django integration`_.
 
 .. _`Celery User Manual`: http://docs.celeryproject.org/
+.. _`subsection about the Django integration`:
+   http://docs.celeryproject.org/en/latest/django/
 .. _`Getting started with django-celery`:
    http://docs.celeryproject.org/en/latest/django/first-steps-with-django.html