Document how to manually stop bitmask-helper
in the transition from 0.20.1 we're getting some issues with the installer failing and leaving things in an unusable state. it can be useful for people testing to be able to manually stop the helper so that they can Trash RiseupVPN.app properly.
it's just launchctl load/unload, but a quick README.testing-osx or something might be helpful to give people to try.