README Improvements

I know the README is WIP re: comment at the bottom, but here is some inspiration for things we can do when it's being rewritten:

- [ ] Compare `help` output from Cobra to Serpent. If we had screenshots side-by-side, that would be very helpful. As I recall, this was one of the primary drivers for creation.
- [ ] Add a "Migrating" section. Assumingly, we want people to migrate their CLI from Cobra if they'd gain the benefits. I _think_(?) we did this incrementally, but I may be incorrect.
- [ ] A logo would be nice for some flashiness. I'm happy to create one when the time arises.
- [ ] Explicitly state that this replaces Cobra and Viper. Viper was created to solve the lack of an environment and YAML config, but in our experience, it falls short.
- [ ] More examples. It might be worth pointing directly to `coder/coder` for an example, since that CLI is massive.