Chapter 3 - Add explainer on the difference between add and put

[matthew-andrews]

https://github.com/matthew-andrews/workshop-making-it-work-offline/tree/master/03-offline-todo/05-adding-data

• https://developer.mozilla.org/en-US/docs/Web/API/IDBObjectStore.add
• https://developer.mozilla.org/en-US/docs/Web/API/IDBObjectStore.put