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>

src/extern/utf8decoder.c

@@ -1,24 +1,11 @@
 /*
- * UTF-8 decoder copyright © 2008–2009 Björn Höhrmann <bjoern@hoehrmann.de>
- * http://bjoern.hoehrmann.de/utf-8/decoder/dfa/
+ * Copyright (c) 2008-2009, Björn Höhrmann <bjoern@hoehrmann.de>
  *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to
- * deal in the Software without restriction, including without limitation the
- * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
- * sell copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
- * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
- * IN THE SOFTWARE.
+ * SPDX-License-Identifier: MIT
+ */
+
+/*
+ * UTF-8 decoder: http://bjoern.hoehrmann.de/utf-8/decoder/dfa/
  */
 
 #include <stdint.h>