add fsharp samples

sheets/_fsharp/function

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+// simple function
+
+// functions in F# are created with `let` keyword as functions are just regular bindings
+
+// sum two values a and b
+let myFunction a b = 
+	// the last value in scope is returned
+	a + b

sheets/_fsharp/hello

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+// hello world
+
+printfn "Hello World!"

sheets/_fsharp/module

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+// in F# module help you to organize your code, like in python or js, a bit like static classes in C# or Java
+
+module MyModule = 
+
+    // bindings in modules are by default public
+    let SOME_CONST = 5
+
+    // same for function bindings
+    let someFun a = 
+        a + SOME_CONST
+
+
+
+// you can access your module somewhere else in your code as such
+let result = MyModule.someFun 3

sheets/_fsharp/open

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+// you can open namespaces with the open keyword
+// a bit like importing packages in other languages
+open System
+
+
+// for example, now wait for a key press and read it, from the System package, Console `module` or static class
+let k = Console.ReadKey()

sheets/_fsharp/rnuget

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+// reference a nuget package in interactive mode! and open its declaring namespace
+
+#r "nuget: FSharp.Data"
+open FSharp.Data