Add reference to Self in traits chapter (book)

Author: munyari

src/doc/book/traits.md

@@ -47,6 +47,28 @@ As you can see, the `trait` block looks very similar to the `impl` block,
 but we don’t define a body, only a type signature. When we `impl` a trait,
 we use `impl Trait for Item`, rather than only `impl Item`.
 
+`Self` may be used in a type annotation to refer to an instance of the type
+implementing this trait passed as a parameter. `Self`, `&Self` or `&mut Self`
+may be used depending on the level of ownership required.
+
+```rust
+trait HasArea {
+    fn area(&self) -> f64;
+
+    fn is_larger(&self, &Self) -> bool;
+}
+
+impl HasArea for Circle {
+    fn area(&self) -> f64 {
+        std::f64::consts::PI * (self.radius * self.radius)
+    }
+
+    fn is_larger(&self, other: &Self) -> bool {
+        self.area() > other.area()
+    }
+}
+```
+
 ## Trait bounds on generic functions
 
 Traits are useful because they allow a type to make certain promises about its