Additional migration notes
Additional migration notes

Summary: Extended migration notes a bit based on feedback of the process so far

Reviewed By: passy

Differential Revision: D28025556

fbshipit-source-id: a3743a76c8a91b16cc3e669494de07baaa8199e6

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper