Update Differences-from-Haskell.md (#149)

Add DataKinds and KindSignatures to the list of PureScript features which resemble Haskell extensions, with a note explanining the difference between user-defined kinds in Haskell vs PureScript.

Author: kcsongor

language/Differences-from-Haskell.md

@@ -292,17 +292,21 @@ In PureScript, operator sections look a little bit different.
 
 The PureScript compiler does not support GHC-like language extensions. However, there are some "built-in" language features that are equivalent (or at least similar) to a number of GHC extensions. These currently are:
 
+* DataKinds (see note below)
 * EmptyDataDecls
 * ExplicitForAll
 * FlexibleContexts
 * FlexibleInstances
 * FunctionalDependencies
+* KindSignatures
 * MultiParamTypeClasses
 * PartialTypeSignatures
 * RankNTypes
 * RebindableSyntax
 * ScopedTypeVariables
 
+Note on `DataKinds`: Unlike in Haskell, user-defined kinds are open, and they are not promoted, which means that their constructors can only be used in types, and not in values. For more information about the kind system, see https://github.com/purescript/documentation/blob/master/language/Types.md#kind-system
+
 ## `error` and `undefined`
 
 For `error`, you can use `Control.Monad.Eff.Exception.Unsafe.unsafeThrow`, in the `purescript-exceptions` package.