Add licensing information

License.txt

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+Public domain
+
+C# port + code by Christian Winnerlein (CodesInChaos)
+
+Poly1305 in c
+        written by Andrew M. (floodyberry)
+        original license: MIT or PUBLIC DOMAIN
+        https://github.com/floodyberry/poly1305-donna/blob/master/poly1305-donna-unrolled.c
+
+Curve25519 and Ed25519 in c
+        written by Dan Bernstein (djb)
+        public domain
+        from Ref10 in SUPERCOP http://bench.cr.yp.to/supercop.html
+
+(H)Salsa20 in c
+        written by Dan Bernstein (djb)
+        public domain
+        from SUPERCOP http://bench.cr.yp.to/supercop.html
+
+SHA512
+        written by Christian Winnerlein (CodesInChaos)
+        public domain
+        directly from the specification