Add Slack badge to README.md

Author: Emmanouil Konstantinidis

README.md

@@ -1,4 +1,4 @@
-# [DRF Docs](http://www.drfdocs.com/) [![travis][travis-image]][travis-url] [![codecov][codecov-image]][codecov-url] [![pypi][pypi-image]][pypi-url]
+# [DRF Docs](http://www.drfdocs.com/) [![travis][travis-image]][travis-url] [![codecov][codecov-image]][codecov-url] [![pypi][pypi-image]][pypi-url] [![slack][slack-image]][slack-url]
 
 Document Web APIs made with Django Rest Framework. [View Demo](http://demo.drfdocs.com/)
 
@@ -74,3 +74,6 @@ First of all thanks to the [Django](http://www.djangoproject.com/) core team and
 
 [codecov-image]: https://codecov.io/github/ekonstantinidis/django-rest-framework-docs/coverage.svg?branch=master
 [codecov-url]:https://codecov.io/github/ekonstantinidis/django-rest-framework-docs?branch=master
+
+[slack-image]: https://img.shields.io/badge/slack-python--dev/drfdocs-e01563.svg
+[slack-url]: https://pythondev.slack.com