docs(dendrite): delete the alias of an empty room

compose/dendrite/README.md

@@ -0,0 +1,13 @@
+# Dendrite Matrix Server
+
+## Notes
+
+### Delete the alias of an empty room
+- Open an SQL shell in the postgres container: `sudo docker exec -it <postgres container id> psql -U dendrite dendrite`
+- List the tables: `\dt`
+- Take note of the aliases table name (it should be `roomserver_room_aliases`)
+- Verify the alias exists: `SELECT * FROM roomserver_room_aliases WHERE alias = '#<alias>:<domain>';`
+  - This should return a row, if it doesn't check for any spelling mistakes
+- Delete the entry from the table: `DELETE FROM roomserver_room_aliases WHERE alias = '#<alias>:<domain>';`
+  - This should return `DELETE 1`
+- Exit the shell: `\q`