Update README with instructions to install OpenOCD on macOS

OpenOCD is required to flash and run the `pw test` command on the
STMicro development board

Change-Id: Iac6a8c24b37699f6693be3bb2e6a6b9a7c834e26

README.md

diff --git a/README.md b/README.md
index 3ede2de..eaff3c4 100644
--- a/README.md
+++ b/README.md
@@ -55,6 +55,13 @@
 And do the following to test on the STM32F429 Discovery board. (The bazel build
 does not yet support building for hardware.)
 
+To flash firmware (and run `pw test`) to a STMicroelectronics Discovery development
+board command you need to install OpenOCD.
+
+On macOS
+install Homebrew using the latest instructions at https://brew.sh/
+Install OpenOCD with `brew install openocd`
+
 ```bash
 $ gn gen --args='pw_target_config = "//targets/stm32f429i-disc1/target_config.gni"' out/disco
 $ ninja -C out/disco