Clarify JTAG disabling steps in getting started doc
Clarify JTAG disabling steps in getting started doc

The old instructions made it seem like MRI only worked with the mbed
device.  This update discusses that JTAG must be disabled for the MRI
debug monitor to run and gives an overview of how this is done on
devices like the mbed, LPCXpresso, etc.

adamgreen/mri,adamgreen/mri,adamgreen/mri