Yes Ahri ADC works very well due to high escape ability with the ultimate Spirit Rush and the kiss ( Charm) negate almost every champ (and worderful if support got grab or crowd control) forcing the ennemy adc to stand away from you always stay behind his creeps.

For the runes, use normal runes : 3 Greater Quintessence of Attack Damage, 9 Greater Mark of Armor Penetration, 9 Greater Seal of Armor, 9 Greater Glyph of Magic Resist.
No need any change since Ahri start with very low rate of armor (but new patch will add her 4 armor but armor runes will be nerfed).

21 9 0 as always it's still the best option for damages and survivability.

For the items, I'll start first with a Doran's Blade and 1 Health Potion then I generally go for a Blade of the Ruined King or Infinity Edge for maximize damage (it depend if support is tanky or if opponent adc go for attack speed, if yes I go for Blade of the Ruined King first).
For the rest follow build description.

Charm is your best skill, the Q ( Orb of Deception) do more damage than Vayne's passive (and more if the kissed is charmed <3) and Spirit Rush is 3 Flash in a row each 1:30 minutes.
Maximize the E ( Charm), then the Q ( Orb of Deception) for True damages and then the W ( Fox-Fire) that deal magic damages in last. Of course ultimate points are to place in Spirit Rush at lvl 6, 11 & 16.

Go for Flash and Barrier as all adc.

+ Great escape (3 flash in a row) ;

- Squishy (low armor) ;

+ Kiss disable almost all adc in 1vs1 ;

- Magic damages ;

+ Got a crowd control at lvl 1 ;

- Spells doesn't stack with AD.

+ Ulti can be used for engage or disengage (or both since you can use it 3 times).

As all ADC, you need to farm on your bot lane for get stronger.