Add hyperlink on documentation in README.md

Author: khankuan

README.md

@@ -111,7 +111,7 @@ py.test graphene_django --cov=graphene_django # Use -v -s for verbose mode
 
 ### Documentation
 
-The documentation is generated using the excellent [Sphinx](http://www.sphinx-doc.org/) and a custom theme.
+The [documentation](http://docs.graphene-python.org/projects/django/en/latest) is generated using the excellent [Sphinx](http://www.sphinx-doc.org/) and a custom theme.
 
 The documentation dependencies are installed by running: