Clarify user-creation during installation

To get the admin user to appear, I had to run the db:seed command and provide the UserSeeder class - so this change clarifies that you should run that command.

Author: rossjones

app/controllers/docs/5.6/installation.md

@@ -107,9 +107,13 @@ This will publish some data sets to some self-explanatory uri's. For example CSV
 
 #### Get started with our user interface
 
-We've created a user interface to manage datasets and users on the uri api/admin, relative to the root uri. The default credentials are:
+We've created a user interface to manage datasets and users on the uri api/admin, relative to the root uri. To create an initial admin user you should run the following command:
+
+    php artisan db:seed --class=UserSeeder
+
+The default credentials for the newly created user are:
 
     user: admin
     password: admin
 
-Best practice is to change the admin password immediately by editing it using the user interface. Click the question mark <i class='fa fa-lg fa-question-circle'></i> in the user interface to start the help guide.
+Best practice is to change the admin password immediately by editing it using the user interface. Click the question mark <i class='fa fa-lg fa-question-circle'></i> in the user interface to start the help guide.