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>

docs/gbz80.7.html

@@ -1689,7 +1689,7 @@ Flags: See <a class="Sx" title="Sx" href="#XOR_A,r8">XOR A,r8</a>
   <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</div>
 <table class="foot">
   <tr>
-    <td class="foot-date">April 17, 2017</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>

docs/rgbasm.1.html

@@ -138,7 +138,7 @@ The resulting object file is not yet a usable ROM image — it must first
   <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</div>
 <table class="foot">
   <tr>
-    <td class="foot-date">April 17, 2017</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>

docs/rgbasm.5.html

@@ -1451,7 +1451,7 @@ The options that OPT can modify are currently: <b class="Sy" title="Sy">b</b>,
   <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</div>
 <table class="foot">
   <tr>
-    <td class="foot-date">January 7, 2018</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>

docs/rgbds.5.html

@@ -298,7 +298,7 @@ Expressions in the object file are stored as RPN. This is an expression of the
   <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</div>
 <table class="foot">
   <tr>
-    <td class="foot-date">January 7, 2018</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>

docs/rgbds.7.html

@@ -52,12 +52,13 @@ To get a working ROM image from a single assembly source file:
   <dt class="It-ohang"></dt>
   <dd class="It-ohang">2017, Bentley's repository is moved to a neutral name. It
       is now maintained by a number of contributors at
-      <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</dd>
+      <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.
+      2018, codebase relicensed under the MIT license.</dd>
 </dl>
 </div>
 <table class="foot">
   <tr>
-    <td class="foot-date">April 17, 2017</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>

docs/rgbfix.1.html

@@ -179,7 +179,7 @@ The following will duplicate the header (sans global checksum) of the game
   <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</div>
 <table class="foot">
   <tr>
-    <td class="foot-date">April 17, 2017</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>

docs/rgbgfx.1.html

@@ -145,7 +145,7 @@ The following will do nothing:
   <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</div>
 <table class="foot">
   <tr>
-    <td class="foot-date">April 17, 2017</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>

docs/rgblink.1.html

@@ -153,7 +153,7 @@ The resulting bar.gb will not have correct checksums (unless you put them in the
   <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</div>
 <table class="foot">
   <tr>
-    <td class="foot-date">April 17, 2017</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>

docs/rgblink.5.html

@@ -97,7 +97,7 @@ Note: The bank, alignment, address and type of sections can be specified both in
   <a class="Lk" title="Lk" href="https://github.com/rednex/rgbds">https://github.com/rednex/rgbds</a>.</div>
 <table class="foot">
   <tr>
-    <td class="foot-date">April 17, 2017</td>
+    <td class="foot-date">January 26, 2018</td>
     <td class="foot-os">RGBDS Manual</td>
   </tr>
 </table>