Add example on closure and references

[cuihtlauac]

Page: Mutability and Imperative Control Flow
Source: 0it_05_imperative.md

Insert this example (or something similar) in the imperative tutorial.

let counter () =
  let n = ref 0 in
  fun () -> incr n; !n;;

This includes showing how it's used, and how it works.

Credit: https://youtu.be/T_w3OlZ5ASc?t=24m25s

[The-Amoghavarsha]

Thanks for the channel recommendation (indirectly). So I'm guessing it has to go under side effects. Can we just call it, Closures after while loop?