Improve E0599 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0599.md

@@ -9,3 +9,18 @@ let x = Mouth;
 x.chocolate(); // error: no method named `chocolate` found for type `Mouth`
                //        in the current scope
 ```
+
+In this case, you need to implement the `chocolate` method to fix the error:
+
+```
+struct Mouth;
+
+impl Mouth {
+    fn chocolate(&self) { // We implement the `chocolate` method here.
+        println!("Hmmm! I love chocolate!");
+    }
+}
+
+let x = Mouth;
+x.chocolate(); // ok!
+```