add document describing type class system and its relation to GADTs
add document describing type class system and its relation to GADTs

darcs-hash:20051209051008-1a7c6-3b818e60583b6e13d7d3fae1e1c945b98a054574

kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc