Added HelloCDK.java which is as close to a HelloWorld example as possible. The example creates, writes and reads from CDK.
Updated READMEs to show the HelloWorld code.
Updating HelloCDK: README discussion, use local FS.

This adds a section that explains the new example more thoroughly to the
README, and makes a couple of minor changes to the code:
1. Uses DatasetRepositories to get a repo, which looks easier
2. Uses a local FS repo to not require HDFS for the hello world
3. Adds Hello#sayHello to demonstrate it's a real object and has methods

Using DatasetRepositories required adding findbugs annotations jar.