We do not require copyright assignment for contributions to KWSys.
Update the top-level notice to mention "Contributors" explicitly.

Change-Id: Id0bd823b8408b8041584a46c81ab46a8cfbe0c0d

This converts the KWSys license to a pure 3-clause OSI-approved BSD
License.  We drop the previous license clause requiring modified
versions to be plainly marked.  We also update the KWSys copyright to
cover the full development time range.