Update README to document status of TUF integrations, Phase 1, Phase 2 etc.

[nealmcb]

The README expresses nearly everything in the future tense:

Phase 1 of the project will implement the basic TUF framework, but leave out author signing; support for author signed packages (and other targets) will added in phase 2.

I know almost nothing about Haskell or Hackage or Cabal, but it seems from the Cabal documentation that this support is already deployed.

Did that use this code? Or some other repository?

Are both phases implemented? Widely used?

Note: I'm coming from the Python world, where TUF integration is an active process, and mainly interested in what we can learn from the Haskell community and experience. I hope I'm not off-base here....