Changelog: Add tip for debian users.

CHANGELOG.md

@@ -5,7 +5,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [unreleased]
 ### Added
-- Optional SSH access mode.
+- Optional SSH access mode. (Needs `erlang-ssh` package on Debian).
 - [MongooseIM](https://github.com/esl/MongooseIM) http authentication support.
 - LDAP authentication
 - External OAuth provider authentication