Update README.md, add Gitter badge

Author: jonhealy1

README.md

@@ -9,6 +9,8 @@
 
 
   [![PyPI version](https://badge.fury.io/py/stac-fastapi.elasticsearch.svg)](https://badge.fury.io/py/stac-fastapi.elasticsearch)  
+  [![Join the chat at https://gitter.im/stac-fastapi-elasticsearch/community](https://badges.gitter.im/stac-fastapi-elasticsearch/community.svg)](https://gitter.im/stac-fastapi-elasticsearch/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+    
 
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