Document pipeline merge logic

[bgrant0607]

I'm not sure there's a design doc for this, 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/1yqTJdpXP_ytGR9AZTbJWZifrFJASPxFuQUbBgNf6kyE/edit#heading=h.ila4xrvnurus

cc @justinsb @mortent