Add a cookbook showing how to re-export types

[popematt]

Sometimes you may have many schema files, but you want to have a single schema file that serves as the "entry point" to or a public API for the data model defined in the larger set of schemas. It is convenient to be able to do that without having to organize the rest of the schema around that one file.

One way to solve this problem is by creating a schema that doesn't define new types, but it "re-exports" existing types from various locations. This solution might not be obvious, and we should add a cookbook to demonstrate how to do it.