Improve clarity of cargo-hf2 readme

[jc-bm]

"If you'd like to not use sudo, you'll need udev rules." could me made clearer by stating the use of sudo is related to running cargo. The first time through I read it as a reference to the previous line "sudo apt-get install libudev-dev libusb-1.0-0-dev" and having something to do with a local install rather than a system install. It currently reads (to me, at least) like the udev setup is an optional extra rather than a good default. I realised it was important after reading #9.