Add readme and tidy up marshmallow_jsonschema

Author: rwb27

src/labthings/json/marshmallow_jsonschema/README

@@ -3,4 +3,22 @@ This submodule is a modified version of fuhrysteve/marshmallow-jsonschema, with
 This will convert any Marshmallow schema into a JSON schema, without any schema references or additional properties. 
 It will create a basic, inline schema only.
 
+It has also been modified (with the addition of `.base.convert_type_list_to_oneof`) to avoid returning list values for
+the type of a schema (this is valid JSON schema, but is not permitted in Thing Description syntax).  Instead, we expand
+the list, by creating a copy of the schema for each type, and combining them using `oneOf`.  This means that
+`fields.String(allow_none=True)`, which would previously be rendered as:
+```json
+{"type": ["string", "null"]}
+```
+will be dumped as
+```json
+{
+    "oneOf": [
+        {"type": "string"},
+        {"type": "null"}
+    ]
+}
+```
+This is also valid JSONSchema, though clearly less elegant.  However, it's required by the thing description.
+
 https://github.com/fuhrysteve/marshmallow-jsonschema

src/labthings/json/marshmallow_jsonschema/base.py

@@ -79,6 +79,9 @@ def convert_type_list_to_oneof(schema):
     that use a list of types into a series of schemas each with
     one type.  Those schemas are then nested together using
     OneOf.
+
+    Schemas that do not have a type that is a list are returned
+    unmodified.
     """
     if "type" not in schema or type(schema["type"]) != list:
         return schema
@@ -223,8 +226,7 @@ def _get_schema_for_field(self, obj, field):
                 )
                 if base_class is not None and base_class in FIELD_VALIDATORS:
                     schema = FIELD_VALIDATORS[base_class](schema, field, validator, obj)
-        if "type" in schema and type(schema["type"]) == list:
-            schema = convert_type_list_to_oneof(schema)
+        schema = convert_type_list_to_oneof(schema)
         return schema
 
     def _from_nested_schema(self, _, field):