docs: add quick tip for how to impl Arbitrary manually

Closes #242

README.md

@@ -1,3 +1,5 @@
+quickcheck
+==========
 QuickCheck is a way to do property based testing using randomly generated
 input. This crate comes with the ability to randomly generate and shrink
 integers, floats, tuples, booleans, lists, strings, options and results.
@@ -359,6 +361,35 @@ and the whole idea of property checking is to take that burden
 off the programmer. Despite the theoretical discomfort, this
 approach can turn out to be practical.
 
+### Generating Structs
+
+It is very simple to generate structs in QuickCheck. Consider the following
+example, where the struct `Point` is defined:
+
+```rust
+struct Point {
+    x: i32,
+    y: i32,
+}
+```
+
+In order to generate a random `Point` instance, you need to implement
+the trait `Arbitrary` for the struct `Point`:
+
+```rust
+use quickcheck::{Arbitrary, Gen};
+
+impl Arbitrary for Point {
+    fn arbitrary<G: Gen>(g: &mut G) -> Point {
+        Point {
+            x: i32::arbitrary(g),
+            y: i32::arbitrary(g),
+        }
+    }
+}
+```
+
+
 ### Case study: The Sieve of Eratosthenes
 
 The [Sieve of Eratosthenes](http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)