Update README to clarify requirement on IRequest interface

[bifocalpirate]

The interface implementation for

public class CreateContactCommand : IRequest<CreateContactCommand, Task<int>>

and

public class CreateContactCommandHandler : IRequestHandler<CreateContactCommand, Task<int>>

differs from

public class CreateContactCommand : IRequest<CreateContactCommand, Task>

and

public class CreateContactCommandHandler : IRequestHandler<CreateContactCommand, Task>

If I have a Validator implemented as

public class CreateContactCommandValidator : AbstractValidator<CreateContactCommand>

The second interface implementation does not trigger the Handler, while the first does. It seems that every request must always have a return type of Task<T> and not only Task.

Can we update the README to reflect this?