Add command for running tests on hardware.

Change-Id: If6e4ee512bc9cf8817688e630886b27573d9d0b8

README.md

diff --git a/README.md b/README.md
index 359524e..67fa5a3 100644
--- a/README.md
+++ b/README.md
@@ -40,12 +40,13 @@
 $ bazel test //...
 ```
 
-And do the following to test on hardware. (The bazel build does not yet
-support building for hardware.)
+And do the following to test on the STM32F429 Discovery board. (The bazel build
+does not yet support building for hardware.)
 
 ```bash
 $ gn gen --args='pw_target_config = "//targets/stm32f429i-disc1/target_config.gni"' out/disco
 $ ninja -C out/disco
+$ pw test --root out/disco/ --runner stm32f429i_disc1_unit_test_runner -- --port /dev/ttyACM0
 ```
 
 If any of this doesn't work please