Maybe mention JSON Schema's extensibility

The "Extending ASDF" has a section introducing "YAML Schema" as a small extension to "JSON Schema"--this is certainly accurate, but I feel like it might be worth mentioning here that JSON Schema is actually *designed* to allow this sort of extension, and that the use of the `$schema` property to point to a custom meta-schema is allowed by the standard (with the caveat that implementations are not required to understand properties defined by your extension).  

Not sure though--maybe this is too much of a technical point.  I just like it because we can say "Hey, adding on to JSON Schema to make it support YAML isn't a hack! It's a feature!"