NEWS updates for 1.6.2

Add an entry for 1.6.2 with "in progress" instead of a date to hold the
release notes for 1.6.2pre4 rather than creating separate entries for
each release candidate. Users who track the release candidates can refer
to the public announcements, and this will be cleaner when reading
history later on.

Change-Id: I625044e20836e072b62ffa738eca23f1d11813a5
Reviewed-on: http://gerrit.openafs.org/8750
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: Ken Dreyer <ktdreyer@ktdreyer.com>