Add Slack badge.

README.md

@@ -2,6 +2,7 @@
 ![Platform](https://img.shields.io/badge/platform-Android-blue.svg)
 [![License](https://img.shields.io/badge/license-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![Build status](https://circleci.com/gh/opendatakit/collect.svg?style=shield&circle-token=:circle-token)](https://circleci.com/gh/opendatakit/collect)
+[![Slack Status](http://slack.opendatakit.org/badge.svg)](https://opendatakit.org)
 
 ODK Collect is an Android app for filling out forms. It is designed to be used in resource-constrained environments with challenges such as unreliable connectivity or power infrastructure. ODK Collect is part of Open Data Kit (ODK), a free and open-source set of tools which help organizations author, field, and manage mobile data collection solutions. Learn more about the Open Data Kit project and its history [here](https://opendatakit.org/about/) and read about example ODK deployments [here](https://opendatakit.org/about/deployments/).