Add an excerpt about entity flags into the book.

[Exiled1]

Currently we have a pattern that goes something like

#[derive(Component, ...)]
struct PlayerFlag;

that gets used by the plugin to retrieve an entityID for an item that they've added components to. We use this pattern instead of attaching state to the ClientState, I think it would be good to explain that that pattern exists so that developers can choose whether or not their plugin entry structs hold state or not.