README.server: Add section on server security
README.server: Add section on server security

- Add section on server security.  Mention that ISR users have shell
access to the server, and explain how to disable directory indexes.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr