Add post about check-ins live updates

I wrote a thing, I'd love feedback about style & content.

Includes style changes for `<ul>` and `<li>`, before: 

![image](https://cloud.githubusercontent.com/assets/2231765/4392174/ea1e4c2a-440b-11e4-837e-8d76dc0a0dea.png)

after:

![image](https://cloud.githubusercontent.com/assets/2231765/4392185/fdd15f6e-440b-11e4-838b-663750a2b383.png)


attendance (also on the line above this)