README: Point to Spark example
README: Point to Spark example

Spark is our most mature blueprint right now, so we should point users
to this repository as a great example of how to use Kelda.

NetSys/quilt,quilt/quilt,LuiseV/quilt,NetSys/quilt,LuiseV/quilt,NetSys/quilt,quilt/quilt,NetSys/quilt,kelda/kelda,quilt/quilt,kelda/kelda,LuiseV/quilt,NetSys/quilt,kelda/kelda,LuiseV/quilt,quilt/quilt,kelda/kelda,LuiseV/quilt