Fix example in README

The example in the README would fail to compile due to an error:
```
  --> src/main.rs:15:1
   |
15 | command!(ping(_context, message) {
   | ^^^^^^^

error: aborting due to previous error
```

This can be fixed by prefixing `extern crate` with `#[macro_use]`.

README.md

@@ -40,7 +40,7 @@ docs.
 A basic ping-pong bot looks like:
 
 ```rust,no-run
-extern crate serenity;
+#[macro_use] extern crate serenity;
 
 use serenity::client::Client;
 use std::env;