add a sentence describing what Patch is to the readme

Author: jmininger

README.md

@@ -1,3 +1,13 @@
 ## Patch
 
 Infrastructure for writing patches which act on other types.
+
+A `Patch` type represents a kind of change made to a datastructure.
+
+```haskell
+class Patch p where
+  type PatchTarget p :: *
+  -- | Apply the patch p a to the value a.  If no change is needed, return
+  -- 'Nothing'.
+  apply :: p -> PatchTarget p -> Maybe (PatchTarget p)
+```