update README

Author: Mika Kunnas

README.md

@@ -102,7 +102,9 @@ If set to `true` and `x-patternProperties` property is present, change `x-patter
 
 #### `patternPropertiesHandler` (function)
 
-Provide a function to handle pattern properties and set `supportPatternProperties` to take effect. The function takes the schema where `x-patternProperties` is defined on the root level. At this point `x-patternProperties` is changed to `patternProperties`.
+Provide a function to handle pattern properties and set `supportPatternProperties` to take effect. The function takes the schema where `x-patternProperties` is defined on the root level. At this point `x-patternProperties` is changed to `patternProperties`. It must return the modified schema.
 
 If the handler is not provided, the default handler is used. If `additionalProperties` is set and is an object, the default handler sets it to false if the root level `type` is the same as one of the types in `patternProperties` unless `type` is `object`. This is because we might want to define `additionalProperties` in OpenAPI spec file, but want to validate against a pattern. The pattern would turn out to be useless if `additionalProperties` of the same type were allowed.
 
+See `test/pattern_properties.js` for examples how this works.
+