Add a note that PolyKinds are not yet fully mature
Add a note that PolyKinds are not yet fully mature

and that bug reports are welcome.

ilyasergey/GHC-XAppFix,ilyasergey/GHC-XAppFix,ilyasergey/GHC-XAppFix,ilyasergey/GHC-XAppFix