Add help for creating a user via docker

changelog.d/7885.doc

@@ -0,0 +1 @@
+Provide instructions on using `register_new_matrix_user` via docker.

docker/README.md

@@ -94,6 +94,21 @@ The following environment variables are supported in run mode:
 * `UID`, `GID`: the user and group id to run Synapse as. Defaults to `991`, `991`.
 * `TZ`: the [timezone](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) the container will run with. Defaults to `UTC`.
 
+## Generating an (admin) user
+
+After synapse is running, you may wish to create a user via `register_new_matrix_user`.
+
+This requires a `registration_shared_secret` to be set in your config file. Synapse
+must be restarted to pick up this change.
+
+You can then call the script:
+
+```
+docker exec -it synapse register_new_matrix_user http://localhost:8008 -c /data/homeserver.yaml --help
+```
+
+Remember to remove the `registration_shared_secret` and restart if you no-longer need it.
+
 ## TLS support
 
 The default configuration exposes a single HTTP port: http://localhost:8008. It