docs: Add an upgrading doc
docs: Add an upgrading doc

Create a new document explaining differences between CC 2.x and 3.x and
what is and is not supported with respect to upgrading.

Fixes #528.

Signed-off-by: James O. D. Hunt <348fbaadfbada55ea105602bf5609e23ac1c4898@intel.com>

devimc/runtime,chavafg/runtime,devimc/runtime,clearcontainers/runtime,clearcontainers/runtime,chavafg/runtime