lguest: document --rng in example Launcher

Rusty Russell wrote:
> Ah, it will appear as /dev/hwrng.  It's a weirdness of Linux that our actual
> hardware number generators are not wired up to /dev/random...

Reflected this in the documentation, thanks :-)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/lguest/lguest.txt

@@ -117,6 +117,11 @@ Running Lguest:
 
   for general information on how to get bridging to work.
 
+- Random number generation. Using the --rng option will provide a
+  /dev/hwrng in the guest that will read from the host's /dev/random.
+  Use this option in conjunction with rng-tools (see ../hw_random.txt)
+  to provide entropy to the guest kernel's /dev/random.
+
 There is a helpful mailing list at http://ozlabs.org/mailman/listinfo/lguest
 
 Good luck!