Create a how-to guide for working with blueprints

[jleibs]

This is the MVP document for getting started with blueprint APIs. Should be a candidate for linking from the release notes.

Overview of the big concepts:

• Containers
• Content expressions

Step-by-step code example for setting up the blueprint.

Would be nice to start with an application that we know has bad heuristics and show how blueprint can provide a better experience.

[jleibs]

Reminder to self: make sure docs cover when you need to use unique app-ids (such as when blueprint is a function of data)