README.md: Add notes about iopl and AC adapter requirement

In order for the EC to maintain the state of whether or not to set the
flash descriptor override across a power cycle, the AC adapter must be
connected, as the system leaves the voltage rail that the EC uses
powered under this condition. Without this, the utility may fail,
continually asking the user to power off and on.

On Linux, CONFIG_X86_IOPL_IOPERM must be set for the kernel, or else the
iopl call will error with "Function not implemented". Make a note of
this in case a user runs into this issue.

Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>

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