I honestly have yet to see a good Sejuani, let alone a good Jungle Sejuani. The reason for this, in my experience, is that Sejuani actually gets kills. As you said, she doesn't have very good damage. However, she's a natural kill-stealer because Northern Winds does just enough damage to finish off low-health champs around her before the carries have a chance to do so. In other words, she gets relatively fed without the capability to abuse it, and prevents her carries from getting fed.

You should add a more comprehensive strategy section, complete with ganking, jungling, and more in-depth team fight descriptions. Perhaps a section dedicated to Northern Winds would be helpful, describing when to activate it and when not to activate. When trying to feed your carries (part of your job as the tank), timing is key. Knowing when to activate Permafrost, when to turn on Northern Winds, as well as the decision of whether to initiate with Arctic Assault and Glacial Prison, separates the bad from the good. Without this mastery of the timing of her abilities, Sejuani becomes a bigger kill-stealer than tanky Garen.

Also, I would keep Philosopher's Stone around a little longer. You'll need the gold/5, since you'll not be getting the kills in your tank-team relationship.