Create THIRD-PARTY-NOTICES.TXT (#6831)

A TPN for https://github.com/dotnet/fsharp/blob/ec04e47bb0b63495697459411b42cc8e126b7be8/tests/fsharp/core/members/ops/test.fsx#L394

Author: richlander

tests/fsharp/core/members/ops/THIRD-PARTY-NOTICES.TXT

@@ -0,0 +1,24 @@
+F# uses third-party libraries or other resources that may be
+distributed under licenses different than the .NET Core software.
+
+Attributions and license notices for test cases originally authored by
+third parties can be found in the respective test directories.
+
+In the event that we accidentally failed to list a required notice, please
+bring it to our attention. Post an issue or email us:
+
+           dotnet@microsoft.com
+
+The attached notices are provided for information only.
+
+License notice for Stack Overflow
+-------------------------------------
+
+Policy: https://stackoverflow.com/help/licensing
+License: https://creativecommons.org/licenses/by-sa/3.0/
+
+Content: How to write a variadic function in F# emulating a similar Haskell solution?
+URL: https://stackoverflow.com/questions/28243963/how-to-write-a-variadic-function-in-f-emulating-a-similar-haskell-solution/28244413#28244413
+
+Question author: robkuz -- https://stackoverflow.com/users/621427/robkuz
+Answer author: Gustavo -- https://stackoverflow.com/users/446822/gustavo