message: Update doc of webapp.
diff: @@ -19,6 +19,10 @@ use rspotify::blocking::util;
  use std::env;
  use std::collections::HashMap;
  +/// In this example, the token is saved into a cache file. If you are building a real-world web
  +/// app, it's easy to save token into database, by calling the function
  +/// `util::get_token_without_cache()`, instead of `util::get_token()`, which saves token by
  +/// default.
  #[derive(Debug, Responder)]
  pub enum AppResponse {
  