Document how to add custom type in event decoding

[h4x3rotab]

Suppose we have an customized event defined in a pallet:

pub enum Event<T> ... {
  MyEvent(MyStruct),
}
#[derive(...)]
struct MyStruct {
  field1: u32
}

Once I send and watch any extrinsic with subxt, it tries to decode all the events. As long as there's one or more events with customized type, it fails with Err(TypeSizeUnavailable("MyStruct")). After reading some code, I finally fixed it by adding a associated type in the runtime trait like this:

impl MyModule for MyRuntime {
    type MyStruct = MyStruct;
}

#[module]
pub trait MyModule: System + Balances {
  type MyStruct: Encode + Decode + PartialEq + Eq + Default + Send + Sync + 'static;
}

I think it would be nice to document it. Also, I wonder why we need such a long list of constraint for the associated type.

[ascjones]

Closed by #294