Files and their performance

We should have a page that describes the performance considerations of file IO in asynchronous applications. In particular we should discuss the `File` object, the `fs::read` function and using the std `File` in `spawn_blocking`.

Related: [tokio#2700][1].

[1]: https://github.com/tokio-rs/tokio/issues/2700