docs: add note about current draft support

Author: kylef

README.md

@@ -1,7 +1,10 @@
 # JSON Schema
 
 An implementation of [JSON Schema](http://json-schema.org/) in Swift.
-Supporting JSON Schema Draft 4, 6 and 7.
+Supporting JSON Schema Draft 4, 6, 7, 2019-09, 2020-12.
+
+The JSON Schema 2019-09 and 2020-12 support are incomplete and have gaps with
+some of the newer keywords.
 
 JSONSchema.swift does not support remote referencing [#9](https://github.com/kylef/JSONSchema.swift/issues/9).