Adding diagram to the Data section in the documentation

After explaining the relationship between publish/subscribe and collection yet again, I'm convinced some diagrams could help some people grok the fundamentals faster and more easily. So I just made a diagram.

I'm also attaching an alternative I made, in case you like that one more... LOL
![Copy of Meteor Collections Diagram](https://f.cloud.github.com/assets/125881/325549/ea1d4760-9b0d-11e2-9294-a2a711d01e29.png)