add auto join option to dendrite config

roles/custom/matrix-dendrite/templates/dendrite/dendrite.yaml.j2

@@ -414,3 +414,16 @@ tracing:
 # Logging configuration, in addition to the standard logging that is sent to
 # stdout by Dendrite.
 logging: []
+
+# Users who register on this homeserver will automatically be joined to the rooms listed under "auto_join_rooms" option.
+# By default, any room aliases included in this list will be created as a publicly joinable room
+# when the first user registers for the homeserver. If the room already exists,
+# make certain it is a publicly joinable room, i.e. the join rule of the room must be set to 'public'.
+# As Spaces are just rooms under the hood, Space aliases may also be used.
+#auto_join_rooms:
+#  - "#main:matrix.org"
+
+{% if matrix_dendrite_auto_join_rooms|length > 0 %}
+auto_join_rooms:
+{{ matrix_dendrite_auto_join_rooms|to_nice_yaml(indent=2, width=999999) }}
+{% endif %}