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>

CONTRIBUTORS.rst

@@ -0,0 +1,39 @@
+Contributors to RGBDS
+=====================
+
+Original author
+---------------
+
+- Carsten Elton Sørensen <csoren@gmail.com>
+
+Main contributors
+-----------------
+
+- Justin Lloyd <jlloyd@imf.la>
+
+- Vegard Nossum <vegard.nossum@gmail.com>
+
+- Anthony J. Bentley <anthony@anjbe.name>
+
+- stag019 <stag019@gmail.com>
+
+- Antonio Niño Díaz <antonio_nd@outlook.com>
+
+Other contributors
+------------------
+
+- Ben10do
+
+- Björn Höhrmann <bjoern@hoehrmann.de>
+
+- Christophe Staïesse <chastai@skynet.be>
+
+- The Musl C library <http://www.musl-libc.org>
+
+- The OpenBSD Project <http://www.openbsd.org>
+
+- Sanqui <gsanky@gmail.com>
+
+- YamaArashi <shadow962@live.com>
+
+- yenatch <yenatch@gmail.com>