clarify the license and copyright terms of the project

As has been the case, contributions are copyright their respective
contributors and the project is BSD-2 licensed. By contributing to the
project, contributors release their contributions under these copyright
and license terms as declared in LICENSE.

CONTRIBUTORS.md

@@ -15,3 +15,5 @@ to see line-by-line credits and
 to see the change log even across renames and rewrites.
 
 Please refer to the [acknowledgements](http://caffe.berkeleyvision.org/#acknowledgements) on the Caffe site for further details.
+
+**Copyright** is held by the original contributor according to the versioning history; see LICENSE.

LICENSE

@@ -1,6 +1,22 @@
+COPYRIGHT
+
+All contributions by the University of California:
 Copyright (c) 2014, The Regents of the University of California (Regents)
 All rights reserved.
 
+All other contributions:
+Copyright (c) 2014, the respective contributors
+All rights reserved.
+
+Caffe uses a shared copyright model: each contributor holds copyright over
+their contributions to Caffe. The project versioning records all such
+contribution and copyright details. If a contributor wants to further mark
+their specific copyright on a particular contribution, they should indicate
+their copyright solely in the commit message of the change when it is
+committed.
+
+LICENSE
+
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are met: 
 
@@ -20,3 +36,9 @@ LOSS OF USE, 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.
+
+CONTRIBUTION AGREEMENT
+
+By contributing to the BVLC/caffe repository through pull-request, comment,
+or otherwise, the contributor releases their content to the
+license and copyright terms herein.