Update reference.md
Added `Deref` to the "Special Traits" section
@@ -3622,6 +3622,14 @@ The `Sized` trait indicates that the size of this type is known at compile-time.
The `Drop` trait provides a destructor, to be run whenever a value of this type
is to be destroyed.
+## The `Deref` trait
+
+The `Deref<Target = U>` trait allows a type to implicitly implement all the methods
+of the type `U`. When attempting to resolve a method call, the compiler will search
+the top-level type for the implementation of the called method. If no such method is
+found, `.deref()` is called and the compiler continues to search for the method
+implementation in the returned type `U`.
+
# Memory model
A Rust program's memory consists of a static set of *items* and a *heap*.
