improve README.md on what subsystem and contextbounds proc macros do

node/overseer/overseer-gen/README.md

@@ -23,6 +23,8 @@ declarative.
 being consumed by that particular subsystem. Each of those subsystems is required to implement the subsystem
 trait with the correct trait bounds. Commonly this is achieved
 by using `#[subsystem]` and `#[contextbounds]` macro.
+  * `#[contextbounds(Foo, error=Yikes, prefix=wherethetraitsat)]` can applied to `impl`-blocks and `fn`-blocks. It will add additional trait bounds for the generic `Context` with `Context: FooContextTrait` for `<Context as FooContextTrait>::Sender: FooSenderTrait` besides a few more. Note that `Foo` here references the name of the subsystem as declared in `#[overlord(..)]` macro.
+  * `#[subsystem(Foo, error=Yikes, prefix=wherethetraitsat)]` is a extension to the above, implementing `trait Subsystem<Context, Yikes>`.
 * `error=` tells the overseer to use the user provided
 error type, if not provided a builtin one is used. Note that this is the one error type used throughout all calls, so make sure it does impl `From<E>` for all other error types `E` that are relevant to your application.
 * `event=` declares an external event type, that injects certain events