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/asm.h

@@ -1,9 +1,13 @@
 /*
- * asm.h
+ * This file is part of RGBDS.
  *
+ * Copyright (c) 1997-2018, Carsten Sorensen and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/*
  * Contains some assembler-wide defines and externs
- *
- * Copyright 1997 Carsten Sorensen
  */
 
 #ifndef RGBDS_ASM_ASM_H

include/asm/charmap.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_CHARMAP_H
 #define RGBDS_ASM_CHARMAP_H
 

include/asm/fstack.h

@@ -1,9 +1,13 @@
-/*  fstack.h
+/*
+ * This file is part of RGBDS.
  *
- *  Contains some assembler-wide defines and externs
- *
- *  Copyright 1997 Carsten Sorensen
+ * Copyright (c) 1997-2018, Carsten Sorensen and RGBDS contributors.
  *
+ * SPDX-License-Identifier: MIT
+ */
+
+/*
+ * Contains some assembler-wide defines and externs
  */
 
 #ifndef RGBDS_ASM_FSTACK_H

include/asm/lexer.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_LEXER_H
 #define RGBDS_ASM_LEXER_H
 

include/asm/localasm.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_LOCALASM_H
 #define RGBDS_ASM_LOCALASM_H
 

include/asm/main.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_MAIN_H
 #define RGBDS_MAIN_H
 

include/asm/mymath.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_MATH_H
 #define RGBDS_ASM_MATH_H
 

include/asm/output.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_OUTPUT_H
 #define RGBDS_ASM_OUTPUT_H
 

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
 

include/asm/symbol.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_SYMBOL_H
 #define RGBDS_SYMBOL_H