README: Add blurb on installing msp430-gdb; reorg a bit
README: Add blurb on installing msp430-gdb; reorg a bit

nedwill/msp430-emu-uctf,piratejon/msp430-emu-uctf,cemeyer/msp430-emu-uctf,cemeyer/avr-emu,piratejon/msp430-emu-uctf