Update NEWS for 1.6.6

release notes for OpenAFS 1.6.6

Change-Id: Ifa9340d3c2e2a94f2d98adc85dec62022b9ea446
Reviewed-on: http://gerrit.openafs.org/10359
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>