docs: add minimal hint on --disable-service in RAUC faq
docs: add minimal hint on --disable-service in RAUC faq

The ability to use RAUC in non-D-Bus mode is not very well documented.
Thus we add at least a little hint in the FAQ to mention it despite we
do not want to encourage people to use RAUC that way if not mandatory.

Fixes #227

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc