more explanations

the unsubscribe needs to be explained here. In the API docs it says "will be unsubscribed when appropriate" when using $ store prefix, but it's not clear what does "appropriate" means.   
The API full note: "Any time you have a reference to a store, you can access its value inside a component by prefixing it with the $ character. This causes Svelte to declare the prefixed variable, and set up a store subscription that will be unsubscribed when appropriate."

site/content/tutorial/08-stores/05-custom-stores/text.md

@@ -6,6 +6,8 @@ As long as an object correctly implements the `subscribe` method, it's a store.
 
 For example, the `count` store from our earlier example could include `increment`, `decrement` and `reset` methods and avoid exposing `set` and `update`:
 
+[more explanations here]
+
 ```js
 function createCount() {
 	const { subscribe, set, update } = writable(0);