Document how to update HIE

GHC now has support for generating HIE files. This is a relatively new feature that furthers our goal of supporting better user-facing tooling. However, there is not much current info in the source tree for what a GHC dev should do to maintain this structure. For example, if the surface syntax changes, how should the HIE support change accordingly?

Zubin Duggal has kindly offered to write this up; this ticket is to track progress.