Add a copyright declaration (#157)

Author: qinsoon

COPYRIGHT

@@ -0,0 +1,10 @@
+Copyrights in the MMTk project are retained by their contributors. No
+copyright assignment is required to contribute to the MMTk project.
+
+Some files may include explicit copyright notices and/or license notices.
+For full authorship information, see the version control history.
+
+Except as otherwise noted (below and/or in individual files), MMTk is
+licensed under the Apache License, Version 2.0 <LICENSE-APACHE> or
+<http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+<LICENSE-MIT> or <http://opensource.org/licenses/MIT>, at your option.