docs(cookbook): add sample for externally provided model

[RedTn]

Closes #450

[RedTn]

Might be worth switching to ember-did-change-attrs at some point but didn't now since it's still a WIP.

[cristinawithout]

I don't think you want this example to set rows: A([]) on the table component.

It's likely people will want to reuse the component they create with an external data source in more than one place across their application. Setting it this way will make rows shared across all instances of the table. (So when rows change in one table, it will change in every table.) Should probably set the value somewhere like init().

[RedTn]

@buschtoens
Is this still good to be merged once I fix the merge conflicts? The comment from @cristinawithout has already been addressed in the commits I can't resolve the comment on the ui :(