Add .gitreview

Add .gitreview

git-review [1] makes it much easier to submit changes.  Add a default
configuration file.

[1] https://docs.openstack.org/infra/git-review/usage.html

Reviewed-on: https://gerrit.openafs.org/12884
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit c7c71d2429cf685f3ffad6b2e6d102d900edc197)

Change-Id: I271cfeb6aea888ae40539e248a18131b0affeda8
Reviewed-on: https://gerrit.openafs.org/12901
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>