add doc about JSON schema

parser/src/json/README.md

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+# JSON schema -> llguidance converter
+
+This sub-module converts JSON schema to llguidance grammar.
+
+## Supported JSON schema features
+
+Following JSON schema features are supported.
+
+Core features:
+
+- `anyOf`
+- `oneOf` - not supported right now, use `anyOf` instead
+- `allOf` - intersection of certain schemas is not supported right now
+- `$ref` - within the document only
+- `const`
+- `enum`
+- `type` - both single type and array of types
+
+Array features:
+
+- `items`
+- `prefixItems`
+- `minItems`
+- `maxItems`
+
+Object features:
+
+- `properties` - order of properties is fixed to the order in schema
+- `additionalProperties`
+- `required`
+
+String features:
+
+- `minLength`
+- `maxLength`
+- `pattern`
+- `format`, with the following formats: `date-time`, `time`, `date`, `duration`, `email`, `hostname`, `ipv4`, `ipv6`, `uuid`,
+
+Number features (for both integer and number):
+
+- `minimum`
+- `maximum`
+- `exclusiveMinimum`
+- `exclusiveMaximum`