Add a Debian/Ubuntu subsection

As an alternative to rpm-based systems or "systems packaged as
tar files".  Luckily, the instructions are pretty simple.

Change-Id: I8045763b518465c3ffd7fe3f177fff98146033fb
Reviewed-on: http://gerrit.openafs.org/11579
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>