Add HDFS how-to

[esamanas]

1. Shows basically how to read from HDFS
2. Shows what the HDFS URL looks like, including server
   name and port. Adds a CDH5 compliant way of getting that info.
3. Tells you how to set which java implementation the HDFS client will
   use

[srikris]

I think this example makes it look much more complicated than it is. It can be intimidating to new users.

Can you have an example with a full URL and explain in comments how the URL works. So the code is only a single line but all the other stuff is in comments.