add contributors tag to README (#9194)

Readme.md

@@ -4,6 +4,7 @@
 ![JS Build](https://github.com/internetarchive/openlibrary/actions/workflows/javascript_tests.yml/badge.svg)
 [![Join the chat at https://gitter.im/theopenlibrary/Lobby](https://badges.gitter.im/theopenlibrary/Lobby.svg)](https://gitter.im/theopenlibrary/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Open in Gitpod](https://img.shields.io/badge/Contribute%20with-Gitpod-908a85?logo=gitpod)](https://gitpod.io/#https://github.com/internetarchive/openlibrary/)
+[![contributors](https://img.shields.io/github/contributors/internetarchive/openlibrary.svg)](https://github.com/internetarchive/openlibrary/graphs/contributors)
 
 [Open Library](https://openlibrary.org) is an open, editable library catalog, building towards a web page for every book ever published.