Provide a top-level examples dir, including annotated example meta.yaml

[cisaacstern]

In #137 I've got a schema with some basic validation going for meta.yaml's.

It's not currently very clear to the outside user what that schema is, however, without reading the actual traitlets-based schema definition.

As a follow-on, we can add a top-level examples/ directory, and include some sample meta.yaml's in it, with annotations/comments explaining the fields. These files can be tested against the schema introduced by #137 to ensure they don't get stale. 😄