Add example of using the indexing operator to BTreeMap docs

src/liballoc/collections/btree/map.rs

@@ -97,6 +97,9 @@ use self::Entry::*;
 ///     }
 /// }
 ///
+/// // Look up the value for a key (will panic if the key is not found).
+/// println!("Review for Jane: {}", book_reviews["Pride and Prejudice"]);
+///
 /// // iterate over everything.
 /// for (movie, review) in &movie_reviews {
 ///     println!("{}: \"{}\"", movie, review);