Add standalone derive example

[Gordon01]

Description

Since it's a serialization library, an example of how to use it to produce String is important.
serde_json has this:

    // Some data structure.
    let address = Address {
        street: "10 Downing Street".to_owned(),
        city: "London".to_owned(),
    };

    // Serialize it to a JSON string.
    let j = serde_json::to_string(&address)?;

    // Print, write to a file, or send to an HTTP server.
    println!("{}", j);

This PR adds such an example. As a bonus, this crate can now be used without Client for anything, that implements std::io::Write.

Related Issues

No previous issues reported.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• This is a documentation

[maxmindlin]

Thanks for the contribution! The linting error is unrelated to your changes.

[Gordon01]

Thanks for a quick merge!