luci-app-attendedsysupgrade: add README
luci-app-attendedsysupgrade: add README

The app should be explained.

Signed-off-by: Paul Spooren <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@aparcar.org>

lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,hnyman/luci,hnyman/luci,lbthomsen/openwrt-luci,openwrt/luci,openwrt/luci,openwrt/luci,hnyman/luci,hnyman/luci,openwrt/luci,openwrt/luci,openwrt/luci,hnyman/luci,lbthomsen/openwrt-luci,hnyman/luci,openwrt/luci,openwrt/luci,lbthomsen/openwrt-luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,hnyman/luci