Speaker Notes for hashmap.md (#295)

* Update hashmap.md

* Update hashmap.md

* Update hashmap.md

* Update src/std/hashmap.md

Co-authored-by: gendx <gendx@users.noreply.github.com>

* Update hashmap.md

* Wording

---------

Co-authored-by: gendx <gendx@users.noreply.github.com>
Co-authored-by: Andrew Walbran <qwandor@google.com>

src/std/hashmap.md

@@ -35,6 +35,17 @@ fn main() {
 
 <details>
 
+* `HashMap` is not defined in the prelude and needs to be brought into scope.
+* Try the following lines of code. The first line will see if a book is in the hashmap and if not return an alternative value. The second line will insert the alternative value in the hashmap if the book is not found.
+
+  ```rust,ignore
+  let pc1 = page_counts
+      .get("Harry Potter and the Sorcerer's Stone ")
+      .unwrap_or(&336);
+  let pc2 = page_counts
+      .entry("The Hunger Games".to_string())
+      .or_insert(374);
+  ```
 * Unlike `vec!`, there is unfortunately no standard `hashmap!` macro.
 
 </details>