Added extra details around setting up admin user (#1937)

after running create-admin the user will be prompted to provide extra details and set the admin password

docs/installation.rst

@@ -80,7 +80,7 @@ Follow these few simple steps to install Superset.::
     # Install superset
     pip install superset
 
-    # Create an admin user
+    # Create an admin user (you will be prompted to set username, first and last name before setting a password)
     fabmanager create-admin --app superset
 
     # Initialize the database