Add usage instructions to README about phidgets_drivers

[clalancette]

@tim-fan Thanks for the detailed feedback. I agree that the documentation could be better here.

It turns out that with "modern" phidgets, we cannot have individual binaries. The reason for this is that when you use a phidget VINT hub, you will have multiple devices connected to the hub, but only one USB connection from that hub to the host computer. When that is the case, you can only have one process that accesses the USB connection at a time. So in order to access all of the devices behind the hub, the ROS 2 phidgets driver is designed to load multiple components into one process. We decided not to provide individual binaries to simplify the story.

But we obviously don't have enough information about this in the README. I'm going to do a small pass over the top-level README at https://github.com/ros-drivers/phidgets_drivers/blob/galactic/README.md and add some additional information.

[mintar]

Thank you for your very nice report and the extra details, this kind of feedback from users is very helpful! When you've worked on a project for a long time, it's sometimes not easy to see which parts of the documentation new users struggle with.

@clalancette : Since this concerns ROS2, could you have a look at this?

[clalancette]

This was fixed by #93, so closing this out.