add instructions on how to build `libhdfs` on a linux box

It wasn't apparent to me how to go about building `libhdfs` on a Linux box, so I've decided to add instructions to the readme.  I've tested this on my VM which is running:

```bash
vagrant@vm:~$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 12.04.1 LTS
Release:	12.04
Codename:	precise
```

Would this be useful for others?