Add Slack forum to README (#6773)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6773

Reviewed By: siying

Differential Revision: D21310229

Pulled By: pdillinger

fbshipit-source-id: c0d52d0c51121d307d7d5c1374abc7bf78b0c4cf

README.md

@@ -24,7 +24,7 @@ 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/
+Design discussions are conducted in https://www.facebook.com/groups/rocksdb.dev/ and https://rocksdb.slack.com/
 
 ## License