Add motivation for forking Dramatiq to README

[jokull]

Just an idea. Couldn’t find this anywhere, and I think most people want to know.

[Baekalfen]

Completely agree. It would be nice to also quickly tell the exact difference between the two. Why should I choose Remoulade over Dramatiq and vice versa? Why isn't this merged into Dramatiq?

[spumer]

I think killer feature is ChannelPool.
Dramatiq can use a lot of connections