Add CC0 license.

Based on investigations by Sven Geggus, the majority of contributors to the
original OSM stylesheets are in favour of a CC0 license, or have not
contributed towards the cartography brought forward into this project.

If anyone can identify their copyrightable contributions within this project
that they do not wish licensed under CC0, then I'll happily and immediately
remove them.

LICENSE.txt

@@ -0,0 +1,15 @@
+OpenStreetMap Carto: A global map style for OpenStreetMap maps.
+
+Created by Andy Allan <[email protected]> and contributors.
+Based on original cartography by Steve Chilton and others.
+
+This software and associated documentation files (the "Software") is
+released under the CC0 Public Domain Dedication, version 1.0, as
+published by Creative Commons. To the extent possible under law, the
+author(s) have dedicated all copyright and related and neighboring
+rights to the Software to the public domain worldwide. The Software is
+distributed WITHOUT ANY WARRANTY.
+
+If you did not receive a copy of the CC0 Public Domain Dedication
+along with the Software, see
+<http://creativecommons.org/publicdomain/zero/1.0/>