message: add platformio manual native install instruction to controller README
diff: @@ -64,10 +64,10 @@ but platformio will just say all checks have passed without giving an error if i
  ## Building and testing
  -After installing platformio, you should be able to build and test as follows.
  +After installing platformio, you should be able to build and test as follows:
  ```
  -$ ./test.sh
  +$ ./controller.sh --test
  # This will run a few commands, such as "platformio run" and
  # "platformio test -e native".
  ```
  @@ -83,6 +83,12 @@ e.g. if things don't build for you in `master`, try:
  $ rm -rf .pio/
  ```
  +If you encounter an error such as `Error: Unknown development platform 'native'`, try running
  +
  +```
  +$ platformio platform install native
  +```
  +
  ## Running on the controller
  To run this you will need at a Nucleo dev board and/or some version of the PCB.
  