Adds flag support policy for new flags added in minor versions
This came up as part of the Haskell Foundation Stability Working Group.
The goal is to aid communication and set user expectations appropriately about the availability of compiler flags. Concretely, with new flags added in 9.6.7, prior to this policy it is unclear what a user should expect in terms of the availability of those flags for 9.8,9.10, and 9.12.