add security section to the resource manager docs

Author: Thomas Graves

docs/running-on-kubernetes.md

@@ -12,6 +12,11 @@ Kubernetes scheduler that has been added to Spark.
 In future versions, there may be behavioral changes around configuration,
 container images and entrypoints.**
 
+# Security
+
+Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
+Please see [Spark Security](security.html) and the specific security sections in this doc before running Spark.
+
 # Prerequisites
 
 * A runnable distribution of Spark 2.3 or above.

docs/running-on-mesos.md

@@ -13,6 +13,11 @@ The advantages of deploying Spark with Mesos include:
   [frameworks](https://mesos.apache.org/documentation/latest/frameworks/)
 - scalable partitioning between multiple instances of Spark
 
+# Security
+
+Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
+Please see [Spark Security](security.html) and the specific security sections in this doc before running Spark.
+
 # How it Works
 
 In a standalone cluster deployment, the cluster manager in the below diagram is a Spark master

docs/running-on-yarn.md

@@ -9,6 +9,11 @@ Support for running on [YARN (Hadoop
 NextGen)](http://hadoop.apache.org/docs/stable/hadoop-yarn/hadoop-yarn-site/YARN.html)
 was added to Spark in version 0.6.0, and improved in subsequent releases.
 
+# Security
+
+Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
+Please see [Spark Security](security.html) and the specific security sections in this doc before running Spark.
+
 # Launching Spark on YARN
 
 Ensure that `HADOOP_CONF_DIR` or `YARN_CONF_DIR` points to the directory which contains the (client side) configuration files for the Hadoop cluster.

docs/spark-standalone.md

@@ -8,6 +8,11 @@ title: Spark Standalone Mode
 
 In addition to running on the Mesos or YARN cluster managers, Spark also provides a simple standalone deploy mode. You can launch a standalone cluster either manually, by starting a master and workers by hand, or use our provided [launch scripts](#cluster-launch-scripts). It is also possible to run these daemons on a single machine for testing.
 
+# Security
+
+Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
+Please see [Spark Security](security.html) and the specific security sections in this doc before running Spark.
+
 # Installing Spark Standalone to a Cluster
 
 To install Spark Standalone mode, you simply place a compiled version of Spark on each node on the cluster. You can obtain pre-built versions of Spark with each release or [build it yourself](building-spark.html).