Add a section on how to tune the AFS cache using xstat_cm_test

Add a section on how to tune the AFS cache using xstat_cm_test

The cache tuning section covers how to use xstat_cm_test to measure
the cache miss percentage.

LICENSE BSD

Change-Id: I895dbbecd7e16d44e18fa65e5e1aa1da5c567a06
Reviewed-on: http://gerrit.openafs.org/1308
Tested-by: Jason Edgecombe <jason@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>