Relicensing: update CONTRIBUTING.md

CONTRIBUTING.md

@@ -49,6 +49,11 @@ else. If you are new to the project then we will work with you to ensure
 your contributions are formatted with this style, so please do not think of
 it as a road block if you would like to contribute some code.
 
-The library is licensed under the GNU Lesser General Public License; while
-you will retain copyright on your contributions, all changes to the core
-library must be provided under this common license.
+As a contributor to this project, you agree that all of your contributions
+to deal.II (a) be governed by the <b>Developer Certificate of Origin
+version 1.1</b> and (b) be dual-licensed under the terms of the <b>Apache
+License 2.0</b> with <b>LLVM Exception</b> and the <b>GNU Lesser General
+Public License v2.1 or later</b>; see LICENSE.md for details. The deal.II
+project does not require copyright assignments for contributions. This
+means that the copyright for code contributions in the deal.II project is
+held by its respective contributors.