This documentation tells you how to write packaged apps. All developers, however, should know that the new APIs for packaged apps are being released as a developer preview. This means that they are evolving daily, and anything you read now might be different in the near future. Please keep up to date with the API reference and documentation. If you hit any stumbling blocks, feedback is welcome at #chromium-apps
Note: If you've written packaged apps before, your legacy packaged apps will still work the way they always have, but they won't have access to the new APIs.
The Getting Started guide is a great place to start. It's fast reading; shouldn't take more than 10 minutes to read all three docs. After the Getting Started guide, decide what's most relevant to you. The Fundamentals guide covers the details of the app and data lifecycle, or learn more about good app design by reading MVC Architecture. We've also got lots of sample code in our repository that is linked to directly from the documentation.
If you're familiar with the Chrome extension docs, then the Reference docs should seem familiar. Packaged apps and extensions share a common platform. They can access many of the same APIs, they have the same manifest and permissions format. Many of the reference docs are shared; we've filtered accessibility to docs that aren't shared.