Establish policy for crate/recipe inclusion

[aturon]

As the cookbook grows beyond the Libz Blitz, we'll need to put more thought into how we select crates and examples. Let's try to lay out a foundational policy that can grow along with us.

[aturon]

I've sketched some initial thoughts here.

[budziq]

@aturon I've dumped some more or less useful thought on both issues (this an #302). Sorry It took so long. As it turns out I'm daunted by both well formatted text documents as well as nearly empty ones ;)