add derivative work credits

COPYRIGHT

@@ -0,0 +1,46 @@
+xorshift is a derivative work originating from:
+http:https://xoroshiro.di.unimi.it/
+
+- splitmix64 (http:https://xoroshiro.di.unimi.it/splitmix64.c) 
+"""
+ Written in 2015 by Sebastiano Vigna ([email protected])
+
+ To the extent possible under law, the author has dedicated all copyright
+ and related and neighboring rights to this software to the public domain
+ worldwide. This software is distributed without any warranty.
+
+ See <http:https://creativecommons.org/publicdomain/zero/1.0/>. 
+"""
+
+- xoroshiro128+ (http:https://xoroshiro.di.unimi.it/xoroshiro128plus.c)
+"""
+ Written in 2016 by David Blackman and Sebastiano Vigna ([email protected])
+
+ To the extent possible under law, the author has dedicated all copyright
+ and related and neighboring rights to this software to the public domain
+ worldwide. This software is distributed without any warranty.
+
+ See <http:https://creativecommons.org/publicdomain/zero/1.0/>.
+"""
+
+- xorshift128+ (http:https://xoroshiro.di.unimi.it/xorshift128plus.c)
+"""
+ Written in 2014-2016 by Sebastiano Vigna ([email protected])
+
+ To the extent possible under law, the author has dedicated all copyright
+ and related and neighboring rights to this software to the public domain
+ worldwide. This software is distributed without any warranty.
+
+ See <http:https://creativecommons.org/publicdomain/zero/1.0/>.
+"""
+
+- xorshift1024* (http:https://xoroshiro.di.unimi.it/xorshift1024star.c)
+"""
+ Written in 2014 by Sebastiano Vigna ([email protected])
+
+ To the extent possible under law, the author has dedicated all copyright
+ and related and neighboring rights to this software to the public domain
+ worldwide. This software is distributed without any warranty.
+
+ See <http:https://creativecommons.org/publicdomain/zero/1.0/>.
+"""