Update README with synopsis

README.md

@@ -3,6 +3,23 @@
 Monocats provides mono-functor and similar instances from [mono-traversable] for
 Scala developers using cats.
 
+## Synopsis
+
+Monocats provides mono instances which are similar to their cats counterparts,
+but don't allow the parameterized type to change during mapping, which allows
+instances to be defined for more types.
+
+For example, you can't define a Functor instance for `String`, because `String`
+does not have a parameterized type, and mapping from a `Char` to an `Int` can't
+produce a new `String`. However, you can define a `MonoFunctor` instance,
+because mapping must preserve the `Char` type:
+
+    new MonoFunctor[String] {
+      type Element = Char
+
+      def map(string: String)(f: Char => Char): String = string.map(f)
+    }
+
 ## Contributing
 
 Please see [CONTRIBUTING.md](/CONTRIBUTING.md).