Add speaker notes to the thanks page (#2497)

Part of #1083.

Author: mgeisler

src/thanks.md

@@ -11,3 +11,13 @@ We've had a lot of fun putting the course together. The course is not perfect,
 so if you spotted any mistakes or have ideas for improvements, please get in
 [contact with us on GitHub](https://github.com/google/comprehensive-rust/discussions).
 We would love to hear from you.
+
+<details>
+
+- Thank you for reading the speaker notes! We hope they have been useful. If you
+  find pages without notes, please send us a PR and link it to [issue #1083]. We
+  are also very grateful for fixes and improvements to the existing notes.
+
+</details>
+
+[issue #1083]: https://github.com/google/comprehensive-rust/issues/1083