The Arduino Uno R3 doesn't work as a proper USB HID right out of the box, but there is a plethora of information out there about how to flash custom firmware onto the secondary chip, I am pretty certain using a DFU programmer. As for a full keyboard integration? I haven't come across a person or post that intrepid yet, although I imagine it'd just be registering keystroke events to input events. Good luck! If I am still on later i'll link you up to what I've found in the past.