Design an updated spec for "what is an engine"

[ef4]

this could ultimately just be a document that explains how to move across to more modern techniques

notes: this could tie into the v2 app spec i.e. if we have a standard way to boot an ember app we could include some of the needs that engines have

[ef4]

I think this will get covered at least in part by emberjs/rfcs#938

[ef4]

The remaining work here should fall out of doing explicit service injection and route manager, which are both separately tracked projects.