Add docs for package configuration

[savetheclocktower]

There's nothing in the Launch Manual (at least not that I can find) about package configuration. The best documentation for this is hidden in the API docs and it still uses CoffeeScript for its examples (and doesn't mention that most packages put their config schema in package.json now).

I should write a page about the configuration schema and illustrate (with screenshots) how various data types are represented in settings-view.