Expan on Semigroup's documentation to explain purposes behind newtypes

Author: JordanMartinez

src/Data/Semigroup.purs

@@ -20,6 +20,12 @@ import Type.Data.RowList (RLProxy(..))
 -- | One example of a `Semigroup` is `String`, with `(<>)` defined as string
 -- | concatenation. Another example is `List a`, with `(<>)` defined as
 -- | list concatenation.
+-- |
+-- | There are two other ways to implement an instance for this type class
+-- | regardless of which type is used. These instances can be used by
+-- | wrapping the values in one of the two newtypes below:
+-- | 1. `First` - Use the first argument every time: `append first _ = first`.
+-- | 2. `Last` - Use the last argument every time: `append _ last = last`.
 class Semigroup a where
   append :: a -> a -> a