Update release notes for 1.6.3

As of pre3, we'll support Linux 3.10 clients (at least up to rc3).

Change-Id: I81b783ad563e4a7735638bde2aa77d11ea1ec83c
Reviewed-on: http://gerrit.openafs.org/9959
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>