Ensure that GitHub's "View license" link points to complete license information.

refs https://github.com/zcash/librustzcash/issues/443

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

COPYING.md

@@ -0,0 +1,16 @@
+# License
+
+Licensed under either of
+
+ * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+# Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally
+submitted for inclusion in the work by you, as defined in the Apache-2.0
+license, shall be dual licensed as above, without any additional terms or
+conditions.
+