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/asm/rpn.h

@@ -1,3 +1,11 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 1997-2018, Carsten Sorensen and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
 #ifndef RGBDS_ASM_RPN_H
 #define RGBDS_ASM_RPN_H