[Documentation:Developer] Log-in instructions for any user (#607)

Previously the documentation did not clarify that any user may be logged
in as by using their User ID as both their username and password. This
clarifies that ability and gives an easy way to look up a list of users.

Author: micpap25

_docs/developer/getting_started/vm_install_using_vagrant.md

@@ -400,7 +400,12 @@ Below are quick steps to get everything installed and running.
     | grader | grader | Limited access grader submitty user |
     | student | student | Student submitty user |
 
-4. The VM has the following four courses by default and they are all part of the current semester:
+   Note that there are many more student and grader users on the VM; you may
+   log in as any of them using their **User ID** as the username and password.
+   The easiest way to see the list of users is to log in as an instructor, access
+   a course, and click **Manage Students** or **Manage Graders**.
+
+5. The VM has the following four courses by default and they are all part of the current semester:
 
     * tutorial
     * sample