Document how to delete a bridge properly

Author: radon-at-beeper

README.md

@@ -165,3 +165,18 @@ If you don't want a self-hosted bridge anymore, you can delete it using
 it from the Beeper servers (e.g. any rooms and ghost users it created).
 For official bridges, it will also delete the local data directory with the
 bridge config, database and python virtualenv (if applicable).
+
+Note that deleting a bridge through the Beeper client settings will
+*not* delete the bridge database that is stored locally; you must
+delete that yourself, or use `bbctl delete` instead. The bridge
+databases are stored in `~/.local/share/bbctl/prod` by default.
+However, note that if you use any option that causes the bridge
+database to be stored in a separate location, such as `-l` which
+stores it in the current working directory, then `bbctl delete` will
+*not* delete the bridge database, and you will again have to delete it
+manually.
+
+If you later re-add a self-hosted bridge after deleting it but not
+deleting the local database, you should expect errors, as the bridge
+will have been removed from Matrix rooms that it thinks it is a member
+of.