Update README to emphasize onboading

Interesting Zero (https://github.com/remoteinterview/zero) got traction for the ease to start, which is something Polydev does well (but designed to support both existing apps as well as grow into complex apps).

I think it'd be better to emphasize several recipes & examples. Let the simplicity speak for it, instead of explaining "rationale" specific to the pain-points non-trivial projects have.

For instance, file-based routing is useful, but it isn't until you want to have a `.test.js` file or `middleware.js` does the decision to have an explicit whitelist of entry-points make sense under `/routes` instead of allowing ad-hoc files.