Update README.md

Author: chrispyles

README.md

@@ -5,7 +5,7 @@
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+[![Slack](https://img.shields.io/endpoint?logo=slack&url=https%3A%2F%2Fraw.githubusercontent.com%2Fucbds-infra%2Fotter-grader%2Fmaster%2Fslack-shields.json)](https://join.slack.com/t/otter-grader/shared_invite/zt-3drxla0ox-IWYIREOYmKhwL1XTyGQE4Q)
 
 Otter Grader is a light-weight, modular open-source autograder developed by the Data Science 
 Education Program at UC Berkeley. It is designed to work with classes at any scale by abstracting