Added F# (#26791)

It's just a stub and need to be expanded but I guess F# is one of the few language that was inexistant in the platform for now.

guide/english/fsharp/index.md

@@ -0,0 +1,27 @@
+---
+title: F#
+---
+
+## F# 
+
+F# is a strongly typed language that inherit from the object oriented language (C#) but encourage a functional programmation. It runs on the .NET CLR, and can seamlessly inter-operate with C#.
+It is compatible with visual studio and is officialy supported by Microsoft.
+
+#### Immutability
+
+Variables in F# are by default immutable. That means that their values can't be changed after initialization. Mutability can be forced by the keyword "mut". One of the F# best practices is to just make a copy of the variable you want and change the values you need at the initialization of the newly created variable
+
+#### Records
+
+F# can use objects but it is encouraged to use instead Records. Records are a set of members, and can be seens as structs for people that comes from a C background.
+
+For example : 
+
+<code>
+type People = {
+  Age: int 
+  FirstName: string 
+  LastName: string  
+}
+</code>
+