Add note on DataFrame recomme^Cation over RDD

Author: donnemartin

spark/spark.ipynb

@@ -458,6 +458,8 @@
    "source": [
     "## RDDs\n",
     "\n",
+    "Note: RDDs are included for completeness.  In Spark 1.3, DataFrames were introduced which are recommended over RDDs.  Check out the [DataFrames announcement](https://databricks.com/blog/2015/02/17/introducing-dataframes-in-spark-for-large-scale-data-science.html) for more info.\n",
+    "\n",
     "Resilient Distributed Datasets (RDDs) are the fundamental unit of data in Spark.  RDDs can be created from a file, from data in memory, or from another RDD.  RDDs are immutable.\n",
     "\n",
     "There are two types of RDD operations:\n",