README: De-list slack channel, list Google group (#9387)

Summary:
We are phasing out the slack channel, but keeping the Google
Group email list.

Pull Request resolved: facebook/rocksdb#9387

Test Plan: no code

Reviewed By: riversand963

Differential Revision: D33591265

Pulled By: pdillinger

fbshipit-source-id: 48e45a74753d05611db2c8f4efc4de16a1f50e70

README.md

@@ -25,7 +25,7 @@ The public interface is in `include/`.  Callers should not include or
 rely on the details of any other header files in this package.  Those
 internal APIs may be changed without warning.
 
-Design discussions are conducted in https://www.facebook.com/groups/rocksdb.dev/ and https://rocksdb.slack.com/
+Questions and discussions are welcome on the [RocksDB Developers Public](https://www.facebook.com/groups/rocksdb.dev/) Facebook group and [email list](https://groups.google.com/g/rocksdb) on Google Groups.
 
 ## License