Explain how project tagging works on the homepage

I found the project tagging a bit confusing so I've added some instructions to the homepage regarding that. In the process I've also added a list of all the projects you have if you haven't tagged anything yet during the day. I've found that working with tomatoes I would always go to my profile -> my projects in order to see which tags they have.

No pomodoros:
![screenshot 2014-06-20 16 31 37](https://cloud.githubusercontent.com/assets/151725/3342222/46c2cc8e-f890-11e3-85ab-4763e05eadcb.png)

At least 1 pomodoro:
![screenshot 2014-06-20 16 32 12](https://cloud.githubusercontent.com/assets/151725/3342223/47f8f9f2-f890-11e3-97e2-9ea1956cf2e4.png)