docs: add contributors badge

README.md

@@ -7,6 +7,7 @@
 ![python version](https://img.shields.io/badge/python-3.6%2C3.7%2C3.8-blue?logo=python)
 ![Run tests](https://github.com/mingrammer/diagrams/workflows/Run%20tests/badge.svg?branch=master)
 [![todos](https://badgen.net/https/api.tickgit.com/badgen/github.com/mingrammer/diagrams?label=todos)](https://www.tickgit.com/browse?repo=github.com/mingrammer/diagrams)
+![contributors](https://img.shields.io/github/contributors/mingrammer/diagrams)
 
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