util: Add a README file for the m5 utility.

util: Add a README file for the m5 utility.

This is a fairly comprehensive document which describes how to use the
m5 utility, the various libraries/modules, and the various included
tests.

Change-Id: I63b5c0a50852a57e6d1b2779090308994e5d0f81
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38215
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>