message: Update document about async/blocking version, and how to build rspotify.
diff: //! [dependencies]
  //! rspotify = "0.10.0"
  //! ```
  -//!
  -//! By default, Rspotify uses asynchronous programming with `async` and
  -//! `await`, but the `blocking` feature can be enabled to have access to the
  -//! [blocking](blocking/index.html) module, with non-async methods.
  +//! Rspotify uses [maybe_async](https://docs.rs/maybe-async/0.2.0/maybe_async/)
  +//! crate to switch between different HTTP clients to get different features.
  +//! In our case, `rspotify` supports async and blocking feature by triggering
  +//! `maybe_async` inside `Cargo.toml`, so we don't have maintain two sets of
  +//! code which are almostly same. By default, Rspotify uses asynchronous
  +//! programming with `async` and `await` by leveraging `reqwest` crate by
  +//! using `client-reqwest` feature. And the blocking IO feature can be
  +//! enabled to access with `client-ureq` feature, with non-async methods.
  //!
  //! ```toml
  //! [dependencies]
  //! println!("Response: {:?}", tracks.unwrap());
  //! }
  //! ```
  +//! ## Building
  +//! As mention above, rspotify uses `maybe_async` crate to switch between async
  +//! and blocking client, which is triggered inside `Cargo.toml`, so there is
  +//! something you need to pay attention to when you are trying to build `rspotify`
  +//! ```sh
  +//! # Build with `client-reqwest` feature, the `async` version
  +//! cargo build --features client-reqwest
  +//!
  +//! # Build with `client-ureq` feature, the `blocking` version
  +//! cargo build --no-default-features --features client-ureq
  +//!
  +//! # Noticed that you could not build `rspotify` with all features like this:
  +//! cargo build --all --all-features
  +//! # Because in order to switch between different clients, the different
  +//! # clients have to implement the same methods, so if you build with all
  +//! # features, you'll get `duplicate definitions` error. As every coin has two
  +//! # sides, you could only have one side at a time, not all sides of it.
  +//! ```
  pub mod client;
  mod http;
  