README.md: flip gitter badge to slack (#828)

fix #828

Used static image from shields.io; slack doesn't seem to have native support
for 'badges', and other solutions ( slackin ) are nice in providing a activity
count, but require running a service gateway. In any event, it's a href that works (for now).

Author: Chris Turner

README.md

@@ -3,7 +3,7 @@
 [![Coverage Status](https://coveralls.io/repos/datajoint/datajoint-python/badge.svg?branch=master&service=github)](https://coveralls.io/github/datajoint/datajoint-python?branch=master)
 [![PyPI version](https://badge.fury.io/py/datajoint.svg)](http://badge.fury.io/py/datajoint)
 [![Requirements Status](https://requires.io/github/datajoint/datajoint-python/requirements.svg?branch=master)](https://requires.io/github/datajoint/datajoint-python/requirements/?branch=master)
-[![Join the chat at https://gitter.im/datajoint/datajoint-python](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/datajoint/datajoint-python?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Slack](https://img.shields.io/badge/slack-chat-green.svg)](https://datajoint.slack.com/)
 
 # Welcome to DataJoint for Python!
 DataJoint for Python is a framework for scientific workflow management based on relational principles. DataJoint is built on the foundation of the relational data model and prescribes a consistent method for organizing, populating, computing, and querying data.