add law

Author: emilypi

src/Control/Lens/Setter.hs

@@ -1130,6 +1130,7 @@ icensoring l uv = censor (iover l uv)
 --
 -- @
 -- 'locally' l 'id' a ≡ a
+-- 'locally' l f '.' locally l g ≡ 'locally' l (f '.' g)
 -- @
 --
 -- >>> (1,1) & locally _1 (+1) (uncurry (+))