Added documentation status to README

[skip ci]

Author: apragacz

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://travis-ci.org/apragacz/django-rest-registration.svg?branch=master)](https://travis-ci.org/apragacz/django-rest-registration)
 [![Codecov Coverage](https://img.shields.io/codecov/c/github/apragacz/django-rest-registration/master.svg)](https://codecov.io/github/apragacz/django-rest-registration?branch=master)
 [![PyPi Version](https://img.shields.io/pypi/v/django-rest-registration.svg)](https://pypi.python.org/pypi/django-rest-registration/)
+[![Documentation Status](https://readthedocs.org/projects/django-rest-registration/badge/?version=latest)](https://django-rest-registration.readthedocs.io/en/latest/?badge=latest)
 
 User registration REST API, based on django-rest-framework.