add helpful links

[flynnduism]

Placeholder issue to make some UX additions.

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Add some help info

End users who run the installer could be unfamiliar with the concepts around CNAB, Duffle etc; compared to the bundle authors who have to interact with those tools directly. By design, the duffle-bag app doesn't require a user to know how it works, it just does.

It's not vital, but I think adding some context and helpful links into the installer could help people grasp the spec/tools and point them towards docs and repos should they need to find more info or file an issue.

This helpful info shouldn't clutter what is a pretty simple, functional UI - perhaps a more info modal window or separate panel. Rough mock:

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Feedback Messages

Some of the UI errors make references to Duffle binaries. Ideally these messages should include links to those tools/installers/docs as appropriate.

Examples:

Duffle not found - cannot install bundle. 
You need to [install Duffle](url) before you can proceed.