Riven plays the roll of AD offtank exceptionally well. With all her various escape mechanisms she is good at jumping in a teamfight, doing damage, and then jumping out when she starts to get focused.
If you have any questions, over runes, masteries, summoner spells, and item builds then ask me in discussion, otherwise i will not give any information on why i chose them(until i get the time to write about them that is)

Pros
+numerous escapes
+Passive has great synergy with all her abilities
+Ult gives her increased damage as well as a nuke firing in a cone
+Has a sexy bunny skin :)

Cons
-Very squishy early game and must expect to be harassed early game
-Stun is very short compared to other CC
-Riven has no reliable farming tool without using her passive, which requires her to get within melee range

The only way to safely farm with riven is by dashing in with your E ability, striking the minion, and then getting out with your Q, because Riven is very squishy early game and champions like Darius and Jax can dominate you.

Watch out for Riven players in high elo, they will kite you around in lane, and if they get fed they will change the pace of any teamfight simply by using their ult on your team and dashing into you to stun.

Whenever I get the free time I will expand the guide, if I write anything incorrect in the guide, tell me and I will update it(I will eventually finish it)