Put example in Readme

> The comparison of performance part has me convinced.
>
> Now how do I use this lovely crate? Right, I head over to docs.rs and read through the performance comparison again.
>
> Ooooh, down there are the two functions.

Would it be possible to have a minimal example at the top of the readme that showcases the really sleek API? Something like
```rust
let is_identifier_start = is_xid_start('c');
let is_identifier_continue = is_xid_continue('a');
let is_identifier_continue = is_xid_continue('t');
```

Also, thank you very much for all your awesome work on those lovely Rust projects!