Add detailed license information

See #26

Update README.md to reflect the new license. Add COPYING to explain the
license of the project. Add the 'MIT License' in COPYING.MIT and the
GPLv3 in COPYING.GPL.

COPYING

@@ -0,0 +1,10 @@
+Red Moon is a derivative of Shades (https://github.com/cngu/shades)
+licensed under the MIT license by Chris Nguyen. Some parts of Red Moon
+are contributed under the MIT license. See COPYING.MIT.
+
+Red Moon is licensed under the GNU General Public License version 3,
+or (at your option) any later version. See COPYING.GPL for more
+information.
+
+For more information on the license of specific files, see the header
+included at the top of the file.