Document basic parts of the package design

[bgrant0607]

There's no design doc for the "v1" package design in the repo, I didn't find any issue or PR that discussed it, and I didn't see an explanation of the Kptfile schema on the doc site, so I'm filing this with a link to a copy of the original design. If useful, it could be converted to markdown, but this is quicker for now.

This doc should be world readable with the link. https://docs.google.com/document/d/1rwIJGsq9w9JXev9IJROjoiFXo69Y5vY7L6Weq1S6cbk/edit

cc @justinsb @mortent