Broadway integration is documented but not present

[awoimbee]

Relates to #139
#146 added jetstream support to this repo but without the broadway integration (b2cfee1).

Now OffBroadway.Jetstream.Producer is documented but isn't in this lib !
The simple workaround is to add {:jetstream, "~> 0.0.9"}, as a dependency (it hasn't been updated in 2 years...).

[ndrean]

@awoimbee

Kind of "hidden": Using Broadway with Jetstream says:

In this guide, we are going to focus on the fourth point. 
To learn how to start Jetstream locally with Docker Compose
and then add Gnat and Jetstream to your application, 
see the Starting Jetstream section in [Getting Started guide](https://hexdocs.pm/gnat/getting_started.html).

and Getting Started guide says:

https://hexdocs.pm/gnat/getting_started.html#adding-jetstream-and-gnat-to-an-application

defp deps do
  [
    {:gnat, ...},
    {:jetstream, ...}
  ]
end