Document the purpose of the metaschema patch and instructions for use additional keywords not included in the patch or JSON Schema

[duncandewhurst]

From #8 (comment):

...the idea is to enable validation of the schema itself to catch errors like mispelling of a keyword, for example, and to ensure that additional keywords are documented in a metaschema patch.

If a schema designer wants to add more keywords, they should document them in a patch - either this patch, if the keyword might be useful in other contexts, or in a context-specific patch which could be applied 'on top' of this one.

cc @kd-ods