derived-instances.md

@@ -48,6 +48,7 @@ The development version of GHC adds a top-level declaration on the form:
 ```
 
 
-which produces the same instance as would be produced by a `deriving` clause in a `data` or `newtype` declaration, but allows this derivation to be decoupled from the datatype declaration, e.g. in another module.
+which produces the same instance as would be produced by a `deriving` clause in a `data` or `newtype` declaration, but allows this derivation to be decoupled from the datatype declaration, e.g. in another module
+([ GHC documentation](http://www.haskell.org/ghc/dist/current/docs/users_guide/type-extensions.html#stand-alone-deriving))