Note complication in README

packages/perseus/src/util/parse-perseus-json/README.md

@@ -50,7 +50,8 @@ purposes.
 The `defaultProps` of a widget are used during content editing to initialize
 the widget options. They represent our _current version_ of the initial widget
 configuration that content creators should see when they add a widget to an
-exercise.
+exercise. (Actually, the situation is slightly more complicated than this,
+because widget editors can have default props as well.)
 
 The defaults in the parsers represent the values that should be used for
 _old content_, created before the associated prop was added to the widget.