Resize and cold migrate can be confusing since they are
very similar operations and share mostly the same code paths
but there are some notable differences. This adds a contributor
doc, similar to the evacuate-vs-rebuild contributor doc, to
try and explain things at a high level and provide enough links
so contributors can follow along and see where things fit into
the puzzle.
A sequence diagram is sorely needed for this as well but that will
come in a separate change.
Change-Id: I11b401a3f874226fdc20a0ee0bd518192f70fa1d
Matt Riedemann
1e781f6f34