Defer on Mutations

Hi @robrichard,

We are now almost through with the implementation of the current draft of `@defer`.

In the current spec draft, `subsequentPayloads` are being processed after query but nor after mutation. I know deferring the mutation fields themselves does not make any sense, but the result of a mutation field is again a query on the changed state of the server, so should these parts be deferrable?

Also, should there be a note stating that defer can only be applied to queries?