Update open-url docs to mention NSPrincipalClass
Update open-url docs to mention NSPrincipalClass

We got bit by a nasty bug today - if you change your app's info.plist `NSPrincipalClass` to something other to `AtomApplication` (We set it to NylasApplication via a bad find-replace), everything works except open-url is never fired. Add a small reference to the docs so folks know to check this key.
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