docs: Add missing serde requirement in example

[Stranger6667]

Hello :)

First of all, thank you for creating and maintaining this crate. I enjoy it :)

This PR fixes the documentation for the "Getting started" example inside lib.rs. Without serde in dependencies, the compiler throws the following error:

error[E0463]: can't find crate for `serde`
 --> src/main.rs:5:17
  |
5 | #[derive(Debug, Deserialize)]
  |                 ^^^^^^^^^^^ can't find crate
  |
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

I noted that the similar example in README has serde in place and copied it here :)

[Fishrock123]

Thanks for catching this!