Add a diagram of the Agent lifecycle #23

    /// Creates an instance of an agent.
    fn create(link: AgentLink<Self>) -> Self;

    /// This method called on every update message.
    fn update(&mut self, msg: Self::Message);

    /// This method called on when a new bridge created.
    fn connected(&mut self, _id: HandlerId) {}

    /// This method called on every incoming message.
    fn handle_input(&mut self, msg: Self::Input, id: HandlerId);

    /// This method called on when a new bridge destroyed.
    fn disconnected(&mut self, _id: HandlerId) {}

    /// This method called when the agent is destroyed.
    fn destroy(&mut self) {}