Add example / quickstart for VueX

[samselikoff]

"Finally figured it out, it was very simple but painful. If you are using the fetch API it is critical that you start the server prior to new Vuex(). If you are using Axios, it doesn't make a difference."

We should add this to the guides and also add a VueX example

[bruceglazier]

If you do not do this MirageJS will not intercept any traffic and it will not log anything to the console.