Add the .md extension to the NOTES file for minimongo's documentation.

I went to look into the minimongo code, and found the NOTES documentation file. It's markdown, but unfortunately it's not be interpreted as markdown by github. Adding the .md extension is all that's required.

Before
![screen shot 2013-05-28 at 6 19 40 pm](https://f.cloud.github.com/assets/427516/576337/1f3486bc-c7fe-11e2-8797-b278400ba60e.png)

After
![screen shot 2013-05-28 at 6 18 52 pm](https://f.cloud.github.com/assets/427516/576339/31e08ff4-c7fe-11e2-952e-14f0721cd633.png)