Include README as a crate level documentation

Additionally suggest `cargo add` which will always resolve to the latest
version of `retainer`.

Including README this way will make the README example run during unit
testing phase.

Fixes: #11

README.md

@@ -18,11 +18,11 @@ improvements and I'll get to them as soon as possible :).
 ### Getting Started
 
 This crate is available on [crates.io](https://crates.io/crates/retainer). The
-easiest way to use it is to add an entry to your `Cargo.toml` defining the dependency:
+easiest way to use it is to add an entry to your `Cargo.toml` defining the dependency
+using `cargo add`:
 
-```toml
-[dependencies]
-retainer = "0.3"
+```sh
+$ cargo add retainer
 ```
 
 ### Basic Usage

src/lib.rs

@@ -1,8 +1,4 @@
-//! Very small caching utility with async locking support.
-//!
-//! All interaction in this crate will be done through the `Cache` type,
-//! so please see the the `cache` module for further instructions.
-#![doc(html_root_url = "https://docs.rs/retainer/0.3.0")]
+#![doc = include_str!("../README.md")]
 
 // exposed modules
 pub mod cache;