add libusb-dev install to readme

[jaguardo]

sudo apt-get install libusb-dev

otherwise you will get:

ups/src/lib/usbhid.h:4:10: fatal error: usb.h: No such file or directory
    4 | #include <usb.h>
      |          ^~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/openups.dir/build.make:63: CMakeFiles/openups.dir/src/main.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:76: CMakeFiles/openups.dir/all] Error 2
make: *** [Makefile:130: all] Error 2

[robbel]

Another approach would be to use libusb-compat-0.1, which is not available in the Ubuntu repos, but can be built as described here: https://bugs.launchpad.net/ubuntu/+source/libusb/+bug/991004