Relicense codebase under MIT license

With permission from the main authors [1], most of the code has been
relicensed under the MIT license.

SPDX license identifiers are used so that the license headers in source
code files aren't too large.

Add CONTRIBUTORS.rst file.

[1] https://github.com/rednex/rgbds/issues/128

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

include/extern/utf8decoder.h

@@ -1,3 +1,11 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2018, Antonio Nino Diaz and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
 #ifndef EXTERN_UTF8DECODER_H
 #define EXTERN_UTF8DECODER_H