Mention that `app.cash.burst:burst` is an available artifact

[veyndan]

Having a read through the README, there's no mention of the app.cash.burst:burst artifact. There's only mention of the app.cash.burst:burst-coroutines and app.cash.burst:burst-gradle-plugin artifacts. For those not needing the coroutines version of Burst, it'd be useful to know that one's available.

[JakeWharton]

You shouldn't really need to know that it exists, since the plugin will add it automatically.

We can still mention it somewhere, though.