Update NEWS for 1.6.8

release notes for OpenAFS 1.6.8

Change-Id: I77712d776a2c31eb2c65612768734a4cceaa1d34
Reviewed-on: http://gerrit.openafs.org/10958
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>