Add notes to threads.md

From the discussion in google#63.

src/concurrency/threads.md

@@ -24,3 +24,22 @@ fn main() {
 * Threads are all daemon threads, the main thread does not wait for them.
 * Thread panics are independent of each other.
   * Panics can carry a payload, which can be unpacked with `downcast_ref`.
+
+<details>
+
+Key points:
+
+* Notice that the thread is stopped before it reaches 10 — the main thread is
+  not waiting.
+
+* Use `let handle = thread::spawn(...)` and later `handle.join()` to wait for
+  the thread to finish.
+
+* Trigger a panic in the thread, notice how this doesn't affect `main`.
+
+* Use the `Result` return value from `handle.join()` to get access to the panic
+  payload. This is a good time to talk about [`Any`].
+
+[`Any`]: https://doc.rust-lang.org/std/any/index.html
+
+</details>