Add specific documentation around user management. Closes #1978

- Briefly cover sshcommand internals
- Show several examples for using sshcommand
- Split out docs onto a new page and relink to it where necessary
- Refer to user-auth plugin trigger for scoping commands to specific users.

[ci skip]

docs/advanced-installation.md

@@ -48,13 +48,7 @@ Once dokku is installed, if you are not using the web-installer, you'll want to
 
 Set up a domain and a wildcard domain pointing to that host. Make sure `/home/dokku/VHOST` is set to this domain. By default it's set to whatever hostname the host has. This file is only created if the hostname can be resolved by dig (`dig +short $(hostname -f)`). Otherwise you have to create the file manually and set it to your preferred domain. If this file still is not present when you push your app, dokku will publish the app with a port number (i.e. `http://example.com:49154` - note the missing subdomain).
 
-You'll have to add a public key associated with a username by doing something like this from your local machine:
-
-    $ cat ~/.ssh/id_rsa.pub | ssh dokku.me "sudo sshcommand acl-add dokku $USER"
-
-If you are using the vagrant installation, you can use the following command to add your public key to dokku:
-
-    $ cat ~/.ssh/id_rsa.pub | make vagrant-acl-add
+Follow the [user management documentation](/dokku/deployment/user-management) in order to add users to dokku.
 
 That's it!