Clean up README file

This makes the README file much more concise and too the point.  It generally directs the user to the official website, but also includes a few useful pointers at well to help them get started.

Signed-off-by: William Vaughan <wvaughan@linkedin.com>
DATAFU-11 ReservoirSample does not behave as expected when grouping by a key other than ALL

* For algebraic, clear the reservoir before each use.
* For accumulate, clear reservoir in cleanup and don't null out scoreGen to avoid unnecessary garbage collection.

Signed-off-by: William Vaughan <wvaughan@linkedin.com>