updated readme

README.md

@@ -24,7 +24,8 @@ will need to set the `SCALA_HOME` environment variable to point to where
 you've installed Scala. Scala must be accessible through one of these
 methods on Mesos slave nodes as well as on the master.
 
-To run one of the examples, use `./run <class> <params>`. For example:
+To run one of the examples, first run `sbt/sbt package` to create a JAR with
+the example classes. Then use `./run <class> <params>`. For example:
 
     ./run spark.examples.SparkLR local[2]