Add "broadcast" method to DataFrame

As discussed at https://stackoverflow.com/questions/43984068/does-spark-sql-autobroadcastjointhreshold-work-for-joins-using-datasets-join-op/43994022, it's possible to force broadcast of DataFrame, even if total size is greater than ``spark.sql.autoBroadcastJoinThreshold``.

But this not trivial for beginner, because there is no "broadcast" method (I know, I am lazy ...).

We could add this method, with a WARN if size is greater than the threshold.

(if it's an easy one, I could do it?)