While your build is definitely good, the guide itself could be a little more polished. At the moment it's one giant wall of text with absolutely no pictures. I'm not saying you need to take screenshots from the game if you don't have a program to do it (Snagit is a good choice). However, using BBCode more effectively could definitely help you out a lot here. Two simple codes you could use are:

[ [ Name of Object ] ] to get Riven *Remove spaces

[ icon= name of object size=60 ] to get *Remove spaces

There are a lot mroe codes you could easily learn by looking at one of the guides on this site that teach others how to make a guide.

Hope this helps :D

I want to see more about the strategy! Riven is a hard champion to master so people looking up your guide will want to see some more hints and tips as to how to use the champion :D Also get some pictures and more creative coding in there! I recommend looking at http://www.mobafire.com/league-of-legends/build/making-a-guide-101506, a great guide that nearly every MobaFire user looks at, at least once in their guide-making process. Other then those few things, I loved the guide! Keep on improving!

one thing i noticed immediately was that, you have a total of 10 runes for marks, seals, and glyphs... you can only have 9...

Looks like a fairly solid guide.
btw, check out this thread if you want some tips on how you can add BBCode to your guide, it helped me a lot and I think it might for you too:
http://www.mobafire.com/league-of-legends/forum/mobafire-support/bbcode-guide-783
Whenever a name, item or skill is mentioned, I tend to link it, like this:

That could would result in this: Broken Wings

i'd recommend blood thirster instead of last whisper considering that last whisper is kinda situational item.