Add docs for validation mode.

docs/operations/cli.rst

@@ -10,6 +10,17 @@ following are the command line options that Envoy supports.
 
   *(required)* The path to the :ref:`JSON configuration file <config>`.
 
+.. option:: --mode <string>
+
+  *(optional)* One of the operating modes for Envoy:
+
+  * ``serve``: *(default)* Validate the JSON configuration and then serve traffic normally.
+
+  * ``validate``: Validate the JSON configuration and then exit, printing either an "OK" message (in
+    which case the exit code is 0) or any errors generated by the configuration file (exit code 1).
+    No network traffic is generated, and the hot restart process is not performed, so no other Envoy
+    process on the machine will be disturbed.
+
 .. option:: --admin-address-path <path string>
 
   *(optional)* The output file path where the admin address and port will be written.