CONTRIBUTING.md: clarify signed-off-by advice
CONTRIBUTING.md: clarify signed-off-by advice

Clarify the signed-off-by advice in CONTRIBUTING.md.

Signed-off-by: Hannu Nyman <ab53a3387de93e31696058c104e6f769cd83fd1b@iki.fi>
(cherry picked from commit ff2f536dd1473da59ae968e6294c6813d1093c58)

db260179/openwrt-bpi-r1-luci,Sakura-Winkey/LuCI,Sakura-Winkey/LuCI,RuiChen1113/luci,RuiChen1113/luci,Sakura-Winkey/LuCI,db260179/openwrt-bpi-r1-luci,RuiChen1113/luci,db260179/openwrt-bpi-r1-luci,db260179/openwrt-bpi-r1-luci,RuiChen1113/luci,db260179/openwrt-bpi-r1-luci,db260179/openwrt-bpi-r1-luci,Sakura-Winkey/LuCI,Sakura-Winkey/LuCI,db260179/openwrt-bpi-r1-luci,db260179/openwrt-bpi-r1-luci,RuiChen1113/luci,Sakura-Winkey/LuCI,RuiChen1113/luci,Sakura-Winkey/LuCI,RuiChen1113/luci,Sakura-Winkey/LuCI,RuiChen1113/luci