Add missing publicUrl configuration for Dimension

Discussed in #282 (Github Issue).

roles/matrix-dimension/defaults/main.yml

@@ -104,6 +104,12 @@ matrix_dimension_configuration_yaml: |
       circleci: "mxc://t2bot.io/cf7d875845a82a6b21f5f66de78f6bee"
       jira: "mxc://t2bot.io/f4a38ebcc4280ba5b950163ca3e7c329"
 
+  # Settings for how Dimension is represented to the public
+  dimension:
+    # This is where Dimension is accessible from clients. Be sure to set this
+    # to your own Dimension instance.
+    publicUrl: "https://{{ matrix_server_fqn_dimension }}"
+
   # Settings for controlling how logging works
   logging:
     file: /dev/null