Add Transition Committee Charter
This is taken from a previous document stored privately on our Google Drive. It was created in private because Boardman's resignation was not yet announced. Now, this can be public.
This was approved in a vote in the board-internal email list here: https://groups.google.com/u/2/a/haskell.foundation/g/board-internal/c/Y-WjFXiOKUc; voting continued on an amendment to the charter here: https://groups.google.com/u/2/a/haskell.foundation/g/board-internal/c/aJ5__v_L1RY. Everyone who voted was in support.