Documenting StrictSlash usage 

The user of [mux](https://github.com/gorilla/mux) should be able to know why the **order**  (*before/after* a path definition) of calling the `StrictSlash(bool)` method is important:

    m = mux.NewRouter()
    m.StrictSlash(true) // works for the "/hello" path 
    m.HandleFunc("/hello", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Hello  gorilla !"))
    })
    m.StrictSlash(true) // Setting the strictSlash property after constructing the "/hello" route does not affect the router behavior on it

    http.Handle("/", root)
    http.ListenAndServe(http_port, nil