[README] To substitute mongodb by mongodb-10gen.

Hi, I think I have found a mistake in your README.
You write:

> Install MongoDB. Follow the directions [here](http://www.mongodb.org/display/DOCS/Ubuntu+and+Debian+packages), then:
``` bash
apt-get update
apt-get install mongodb
```

but the direction use the up-to-date packages from the 10gen repository.