Add example for iter chain struct

Author: pickfire

library/core/src/iter/adapters/chain.rs

@@ -6,6 +6,17 @@ use crate::usize;
 ///
 /// This `struct` is created by [`Iterator::chain`]. See its documentation
 /// for more.
+///
+/// # Examples
+///
+/// ```
+/// use std::iter::Chain;
+/// use std::slice::Iter;
+/// 
+/// let a1 = [1, 2, 3];
+/// let a2 = [4, 5, 6];
+/// let iter: Chain<Iter<_>, Iter<_>> = a1.iter().chain(a2.iter());
+/// ```
 #[derive(Clone, Debug)]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]