In any question page, there's "Explanations and References" just under the question. When you practice, it will show only when you answer the question. This is where you , as well as other users, can add any notes related to the question. Clicking "add" button will show a form. If it is a reference or a citation, besides explanation field, you can add the source and the link to that resource. The source and link fields are optional. Also, you, and other users, can vote for the best explanation.