Add speaker notes for CXX page (#1060)

While we don’t have slides showing how to use the crate (#823), we can
at least explain at a high level what the instructor should focus on
when walking through the tutorial.

src/android/interoperability/cpp.md

@@ -9,5 +9,26 @@ The overall approach looks like this:
 
 See the [CXX tutorial][2] for an full example of using this.
 
+<details>
+
+- At this point, the instructor should switch to the [CXX tutorial][2].
+
+- Walk the students through the tutorial step by step.
+
+- Highlight how CXX presents a clean interface without unsafe code in _both languages_.
+
+- Show the correspondence between [Rust and C++ types](https://cxx.rs/bindings.html):
+
+    - Explain how a Rust `String` cannot map to a C++ `std::string`
+      (the latter does not uphold the UTF-8 invariant). Show that
+      despite being different types, `rust::String` in C++ can be
+      easily constructed from a C++ `std::string`, making it very
+      ergonomic to use.
+
+    - Explain that a Rust function returning `Result<T, E>` becomes a
+      function which throws a `E` exception in C++ (and vice versa).
+
+</details>
+
 [1]: https://cxx.rs/
 [2]: https://cxx.rs/tutorial.html