Add speaker note with manual derive example (#2607)

src/methods-and-traits/deriving.md

@@ -26,8 +26,28 @@ fn main() {
 
 <details>
 
-Derivation is implemented with macros, and many crates provide useful derive
-macros to add useful functionality. For example, `serde` can derive
-serialization support for a struct using `#[derive(Serialize)]`.
+- Derivation is implemented with macros, and many crates provide useful derive
+  macros to add useful functionality. For example, `serde` can derive
+  serialization support for a struct using `#[derive(Serialize)]`.
+
+- Derivation is usually provided for traits that have a common boilerplate-y
+  implementation that is correct for most cases. For example, demonstrate how a
+  manual `Clone` impl can be repetitive compared to deriving the trait:
+
+  ```rust,ignore
+  impl Clone for Player {
+      fn clone(&self) -> Self {
+          Player {
+              name: self.name.clone(),
+              strength: self.strength.clone(),
+              hit_points: self.hit_points.clone(),
+          }
+      }
+  }
+  ```
+
+  Not all of the `.clone()`s in the above are necessary in this case, but this
+  demonstrates the generally boilerplate-y pattern that manual impls would
+  follow, which should help make the use of `derive` clear to students.
 
 </details>