Add a comment about updating the `runner` section

[tgross35]

It's easy to miss updating the [target.thumbv7em-none-eabihf] section's header, and the runner configuration won't work, and the resulting output isn't very insightful:

qemu-arm: target/thumbv7em-none-eabihf/release/app: Error mapping file: Invalid argument

So, I added some comments about updating the section header and the qemu -cpu flag. Also moved the [build] section to the top to better match the configuration order.