Link to examples and schema from README
Add $schema to JSON Schemas

It is recommended the JSON Schemas declare that they are JSON schemas, as
opposed to simply some JSON.

I used a specific pointer to the current version, rather than the "latest" to
document which version we wrote it against.
Disallow unspecified properties in JSON

It makes the process of checking for unsanitary markup easier if we disallow
unspecified keys: we are then free to use the schema to find all string values
to check.

Additionally, it prevents users from defining new fields in the API. This could
potentially be a disadvantage.

The tests needed to be updated so that they still supplied valid data.
Reorder schemas for readability

It is easier to relate entries to their enclosing hash if they are not at the
end of lots of closing braces.