Add license file

LICENSE

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+As with the original ctrulib, this library is licensed under zlib. This
+applies to every file in the tree, unless otherwise noted.
+
+    This software is provided 'as-is', without any express or implied
+    warranty.  In no event will the authors be held liable for any
+    damages arising from the use of this software.
+
+    Permission is granted to anyone to use this software for any
+    purpose, including commercial applications, and to alter it and
+    redistribute it freely, subject to the following restrictions:
+
+    1. The origin of this software must not be misrepresented; you
+       must not claim that you wrote the original software. If you use
+       this software in a product, an acknowledgment in the product
+       documentation would be appreciated but is not required.
+    2. Altered source versions must be plainly marked as such, and
+       must not be misrepresented as being the original software.
+    3. This notice may not be removed or altered from any source
+       distribution.