Explain more clearly about ListAdapter

In this [`ListAdapter` guide](https://github.com/codepath/android_guides/wiki/Using-the-RecyclerView#using-with-listadapter), the adapter has to be roughly modified like this:

```java
public ContactsAdapter(List contacts) {
super(DIFF_CALLBACK);
this.mContacts = contacts;
submitList(this.mContacts);
}
```

The guide didn't stress the importance of calling `submitList()` even in the first time `mContacts` is assigned.

I thought that `submitList()` would have to be trigger on subsequent modifications, because if it had been triggered, what would have it compare to? Turn out it does compare to `null`, and you have to `submitList()` that.