Copyright.txt: Add notice of copyright by contributors

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

Change-Id: Id0bd823b8408b8041584a46c81ab46a8cfbe0c0d

Copyright.txt

 KWSys - Kitware System Library
-Copyright 2000-2009 Kitware, Inc., Insight Software Consortium
+Copyright 2000-2016 Kitware, Inc. and Contributors
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -13,10 +13,9 @@ are met:
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.
 
-* Neither the names of Kitware, Inc., the Insight Software Consortium,
-  nor the names of their contributors may be used to endorse or promote
-  products derived from this software without specific prior written
-  permission.
+* Neither the name of Kitware, Inc. nor the names of Contributors
+  may be used to endorse or promote products derived from this
+  software without specific prior written permission.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
@@ -29,3 +28,11 @@ DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+------------------------------------------------------------------------------
+
+The following individuals and institutions are among the Contributors:
+
+* Insight Software Consortium <insightsoftwareconsortium.org>
+
+See version control history for details of individual contributions.