Max her shield first, get cooldown reduction.

Lux is a really good support, you just treat her differently early than if you were carrying. As has been stated, she has amazing harass and control. Often, as support, I don't save my lazer for stragglers, I go ahead and blow it as soon as I've rooted them and try to follow up with AA (ideally, if you've got a good spot in the bush, and you can land Q - immediate AA - R - AA). This deals more damage than most supports can put out, and immediately puts the fight in your favor. Usually, this will result in the enemy ADC just trying to get away, which means that your ADC is getting free damage in, either forcing them out of lane or getting a kill.

She's too aggressive to be a support IMO, and it's required you build her AP or her shield won't do much. I wouldn't rely on a skillshot shield from a support who deals more damage than the ADC.

However, you could play her EARLY game support and get Mejai's, Morellonomicon, DFG, Rab's, and 1 support item + boots.

But TBH Janna is better.