Add documentation

[eze-kiel]

Currently the documentation on Rillrate website is empty. As a Rust newbie, I would love to have more infos, at least about how to get started.

Thanks ! 📈 🦀

[aprowe]

For anyone trying to get the readme example to work, I had to clone the repo and install from master ('trunk')
Since I couldn't find the FlowMode enum in 0.41.0, the latest version from cargo.

    rillrate::install("my-app");
    let tracer = rillrate::prime::Pulse::new(
        "package.dashboard.group.tracer-name",
        rillrate::core::FlowMode::Realtime,
        rillrate::prime::PulseOpts::default().min(0).max(50).higher(true)
    );

Is what my Example looked like with qualified names.

Then I had to visit port 6361 for the dashboard (took me 10 minutes to find the port number -_- )