ACPI: thinkpad-acpi: document keymap gotcha's (v2)

Publish the requirements for keymap changes.  This is a documentation
change, only.

Currently, people look at the thinkpad-acpi default keymaps, and think:
"modifying this is a trivial thing, it can't break systems, and there are
keys defined for foo and bar, but the driver has them as KEY_RESERVED.
Must have been an oversight, let me change it."

And since they never get to see the bug reports, because they are not
really a part of the Linux ThinkPad users community (linux-thinkpad
mailinglist, thinkwiki wiki, thinkpad forums) and laptop users are slow
to complain to distros about any breakages...

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>