add documentation

Author: Ji Yan

docs/running-on-mesos.md

@@ -356,6 +356,16 @@ See the [configuration page](configuration.html) for information on Spark config
     By default Mesos agents will not pull images they already have cached.
   </td>
 </tr>
+<tr>
+  <td><code>spark.mesos.executor.docker.params</code></td>
+  <td>(none)</td>
+  <td>
+    Set the list of custom parameters which will be passed into the <code>docker run</code> command when launching the Spark executor on mesos using docker containerizer. The format of this property is a comma-separated list of
+    key/value pairs. That is they take the form:
+
+    <pre>key1=val1,key2=val2,key3=val3</pre>
+  </td>
+</tr>
 <tr>
   <td><code>spark.mesos.executor.docker.volumes</code></td>
   <td>(none)</td>