[FLINK-15276] Add introduction text about executors in cli.md

docs/ops/cli.md

@@ -40,9 +40,33 @@ The command line can be used to
 
 A prerequisite to using the command line interface is that the Flink
 master (JobManager) has been started (via
-`<flink-home>/bin/start-cluster.sh`) or that a YARN environment is
+`<flink-home>/bin/start-cluster.sh`) or that another deployment target such as YARN or Kubernetes is
 available.
 
+### Deployment targets
+
+Flink has the concept of executors for defining available deployment targets. You can see the
+available executors in the output of `bin/flink --help`, for example:
+
+```
+Options for executor mode:
+ -D <property=value> Generic configuration options for
+ execution/deployment and for the configured executor.
+ The available options can be found at
+ https://ci.apache.org/projects/flink/flink-docs-stabl
+ e/ops/config.html
+ -e,--executor <arg> The name of the executor to be used for executing the
+ given job, which is equivalent to the
+ "execution.target" config option. The currently
+ available executors are: "remote", "local",
+ "kubernetes-session", "yarn-per-job", "yarn-session".
+```
+
+When running one of the `bin/flink` actions, the executor is specified using the `--executor`
+option. 
+
+
+
 * This will be replaced by the TOC
 {:toc}