docs: add entity verify failed error documentation

Author: dougwilson

README.md

@@ -307,6 +307,13 @@ parsed by the middleware. The `status` property is set to `400`, the `type`
 property is set to `'entity.parse.failed'`, and the `body` property is set to
 the entity value that failed parsing.
 
+### entity verify failed
+
+This error will occur when the request contained an entity that could not be
+failed verification by the defined `verify` option. The `status` property is
+set to `403`, the `type` property is set to `'entity.verify.failed'`, and the
+`body` property is set to the entity value that failed verification.
+
 ### request aborted
 
 This error will occur when the request is aborted by the client before reading