added troubleshooting section to agent usage home page

@miketheman @remh - take a glance and let me know what you think!



![screen shot 2014-01-02 at 4 33 41 pm](https://f.cloud.github.com/assets/1970694/1835759/9177288a-73f5-11e3-89e7-2bbf1df668bb.png)