add nmiddleware to redux store code example

Example: redux middleware

A Redux middleware is a function returning a function, which takes next as a
 parameter. Then the inner function returns another function which takes action
as a parameter and finally returns next(action). Here's how it looks like:

function Middleware() {
  return function(next){
    return function(action){
      // do your stuff
      return next(action);
    }
  }
}