testdata: explain omitted test files (#106)

Explain why certain files are omitted from our tests.

Author: jba

jsonschema/testdata/draft2020-12/README.md

@@ -2,3 +2,14 @@
 
 These files were copied from
 https://github.com/json-schema-org/JSON-Schema-Test-Suite/tree/83e866b46c9f9e7082fd51e83a61c5f2145a1ab7/tests/draft2020-12.
+
+The following files were omitted:
+
+content.json: it is not required to validate content fields
+(https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-validation-00#rfc.section.8.1).
+
+format.json: it is not required to validate format fields (https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-validation-00#rfc.section.7.1). 
+
+vocabulary.json: this package doesn't support explicit vocabularies, other than the 2020-12 draft.
+
+The "optional" directory: this package doesn't implement any optional features.