[F#/en] added mutation (#4763)

fsharp.html.markdown

@@ -33,6 +33,21 @@ let myInt = 5
 let myFloat = 3.14
 let myString = "hello"           // note that no types needed
 
+// Mutable variables
+let mutable a=3
+a <- 4 // a is now 4.
+
+// Somewhat mutable variables
+// Reference cells are storage locations that enable you to create mutable values with reference semantics.
+// See https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/reference-cells
+let xRef = ref 10
+printfn "%d" xRef.Value // 10
+xRef.Value <- 11
+printfn "%d" xRef.Value // 11
+
+let a=[ref 0; ref 1] // somewhat mutable list
+a[0].Value <- 2
+
 // ------ Lists ------
 let twoToFive = [2; 3; 4; 5]     // Square brackets create a list with
                                  // semicolon delimiters.