Document that functions are stripped from objects and not restored

[cyberco]

Since objects are stored using JSON.stringify functions are stripped off. They're lost when the object is restored. This basically blocks the option of storing class instances (or other objects with functions) in vuex. Although I think you built a nice library this should be mentioned in the main README.md

[Harm-Nullix]

If I may reference: vuejs/vuex#757 (comment)
Save your classes/methods in code and hydrate them with the data from the store.

Serializable is the key word here