Add example of opaque CountDown type usage

Author: Brandon Matthews

src/timer.rs

@@ -57,6 +57,23 @@ use void::Void;
 /// #     fn wait(&mut self) -> ::nb::Result<(), Void> { Ok(()) }
 /// # }
 /// ```
+///
+/// If you want to use a CountDown timer as an opaque type in a HAL-based driver, you have to add
+/// some type bounds to make sure arguments passed to `start` can be used by the underlying
+/// (concrete) `CountDown::Time` type:
+/// ```rust
+/// extern crate embedded_hal as hal;
+/// #[macro_use(block)]
+///
+/// pub fn uses_timer<T, U>(t: T)
+/// where
+///     T: hal::timer::CountDown<Time=U>,
+///     U: From<u32>,
+/// {
+///     // delay an arbitrary 1 time unit
+///     t.start(1);
+/// }
+/// ```
 pub trait CountDown {
     /// The unit of time used by this timer
     type Time;