update docs

src/formatron/formats/json.py

@@ -442,6 +442,9 @@ def __init__(self, nonterminal: str, capture_name: typing.Optional[str], schema:
             - pattern currently is automatically anchored at both ends
             - the generated json could be invalid if the pattern allows invalid content between the json string's quotes.
               - for example, `pattern=".*"` will allow '\"' to appear in the json string which is forbidden by JSON standard.
+          - also supports substring_of constraint which constrains the string to be a substring of a given string
+            - the generated json could be invalid if the given string contains invalid content when put into the json string's quotes.
+              - for example, `substring_of="abc\""` will allow '\"' to appear in the json string which is forbidden by JSON standard.
         - NoneType
         - typing.Any
         - Subclasses of collections.abc.Mapping[str,T] and typing.Mapping[str,T] where T is a supported type,

src/formatron/schemas/json_schema.py

@@ -53,6 +53,7 @@ def create_schema(schema: dict[str, typing.Any], registry=Registry()) -> schemas
 
     - `type` keyword
     - `minLength, maxLength, pattern` keywords for string type
+    - `substringOf` keyword for string type
     - `minimum, maximum, exclusiveMinimum, exclusiveMaximum` keywords for number type and integer type
     - `items` keyword
       - optionally with `minItems`, `maxItems`, `prefixItems` constraints