arch/xtensa: Add an arch-internal README on register windows

Back when I started work on this stuff, I had a set of notes on
register windows that slowly evolved into something that looks like
formal documentation.  There really isn't any overview-style
documentation of this stuff on the public internet, so it couldn't
hurt to commit it here for posterity.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>