Flesh out README; tweak dump syntax

Got rid of the _ in the syntax for frames, so `f 4` translates to `<f | $ 4>`.
Also, drop the angle brackets and the pipe when the continuation is empty.

In the README, besides revising the examples to match the syntax changes,
mostly I just added a bit about how the Sequent Core types are really just the
Core type split into three.
Prepare sequent-core.cabal for Hackage upload