Update README.md (#4169)

Summary:
Pull Request resolved: #4169

Point users to Github discussions page instead of Facebook group for release updates, Q&A, etc.

Reviewed By: mnorris11

Differential Revision: D69140649

fbshipit-source-id: 56c7ed54411063789ed56ffc6aa23ff6cc696ef3

Author: Amir Sadoughi

README.md

@@ -80,7 +80,7 @@ For the GPU version of Faiss, please cite:
 
 ## Join the Faiss community
 
-For public discussion of Faiss or for questions, there is a Facebook group at https://www.facebook.com/groups/faissusers/
+For public discussion of Faiss or for questions, visit https://github.com/facebookresearch/faiss/discussions.
 
 We monitor the [issues page](http://github.com/facebookresearch/faiss/issues) of the repository.
 You can report bugs, ask questions, etc.