Add spectrum badge to README (#1519)

Author: vincentschen

README.md

@@ -7,6 +7,8 @@
 [![docs](https://readthedocs.org/projects/snorkel/badge/?version=master)](https://snorkel.readthedocs.io/en/master)
 [![coverage](https://codecov.io/gh/snorkel-team/snorkel/branch/master/graph/badge.svg)](https://codecov.io/gh/snorkel-team/snorkel/branch/master)
 [![license](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
+[![Join the community on Spectrum](https://withspectrum.github.io/badge/badge.svg)](https://spectrum.chat/snorkel)
+
 
 ***Programmatically Build and Manage Training Data***