Update README file and add README-LOGIC with tips for the PL

I keep forgetting how to build the HDMI and other stuff into the logic,
so I put a quick howto into a text file.
linux-milo: Implement the GPIO buttons routed through logic

Integrate a fix for the IRQ handler in the kernel. This allows the
buttons on the boards to work as normal keyboard buttons. For example,
the zedboard keys now map to the numeric keypad keys so you can play
games if you attach just a screen...