Add more documentation for testing on host

examples/test_on_host.rs

@@ -1,5 +1,14 @@
 //! Conditionally compiling tests with std and our executable with no_std.
 //!
+//! Rust's built in unit testing framework requires the standard library,
+//! but we need to build our final executable with no_std.
+//! The testing framework also generates a `main` method, so we need to only use the `#[entry]`
+//! annotation when building our final image.
+//! For more information on why this example works, see this excellent blog post.
+//! https://os.phil-opp.com/unit-testing/
+//!
+//! Running this example:
+//!
 //! Ensure there are no targets specified under `[build]` in `.cargo/config`
 //! In order to make this work, we lose the convenience of having a default target that isn't the
 //! host.