I'm curious why you would put two points into Archmage's Savvy . Riven has no AP. I know that one is there for rung advancement, but I'd say that you could move one point from Archmage's Savvy and the two in Offensive Mastery into Sorcery for the CDR. Just a thought.

I'd also recommend trying to make use of the Wikicodes contained in the guide Zedera linked you to.

Could use a lot of expanding. You've got the basics down pretty well. But to hit that next level, why not take a look at this guide? I found it very handy in making my own Riven guide for Dominion, as it helps you to find out how to do all the fancy things that some guides manage to do. Otherwise, not bad at all.

Edit: Just noticed Black Cleaver and Phantom Dancer. Seeing as Riven hardly benefits from AS at all, maybe opt for some better items? Another Infinity Edge never hurt anyone...except the other side...

This is a great build after using this i went 21-2, 35-5 and 19-0, could use more of a guide tho