Describe the correct pairing procedure in README

[spoelstraethan]

One thing I didn't notice in the README was how to pair a new device, I think in one attempt I paired using the UI Bluetooth settings and then restarted rpi-kvm and it was able to utilize that pairing. I think I later realized that when the service starts it sets up the Bluetooth in discoverable mode all the time? But I had trouble trying to pair from the mobile phone side and ended up doing the pairing from the command line on the Raspberry Pi with the phone in discoverable mode. It would be nice to be able to scan and connect and turn the scanning on and off (via the CLI or web) as you may not want the extra RF noise or people attempting to pair unexpectedly. There's a nice pairing script in the pihidproxy repo that just needs a sleep of a couple seconds to let the scan discover ALL the local devices if you are in a noisy airspace, and I've been successfully tested adding a device name filter as an argument to the script to try and connect to a specific device once the name resolves. The biggest challenge with using a script is when you connect to a phone you have to accept the prompt there and then also confirm the pin that was displayed on the phone on the pi.

I also discovered that while you can easily connect and disconnect and select an active device via the web there's not a way to unselect a device to direct inputs to the host OS (like on the Pi400 where I have a Hyperpixel touchscreen LCD), and disconnecting the currently active device is sort of the same thing but not intuitive. On the Pi 400 it also appears that the keyboard and mouse don't get captured exclusively so the inputs go to the Pi UI as well as getting forwarded to the mobile devices.

It would also be nice to filter to only input devices and ignore certain event devices, because on newer raspberry Pi OS installs the HDMI shows up as event0. I would also want to ignore the touch screen as an input device to send to the phones since touchscreens user absolute positioning and it wouldn't be the same resolution as their screens and allowing the touch screen to be used locally would let me open the web page for device selection and switching and activate via the touch screen instead of keyboard shortcuts (which isn't actually working for me right now anyways).