Add passthrough_nodelet documentation

doc/jsk_topic_tools/lib/passthrough_nodelet.md

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+# Passthrough
+
+`jsk_topic_tools/Passthrough` is a node/nodelet to relay topics only for specified duration. You can use the service call to turn relay on and off. By default, the topic relay is turned off.
+
+## Subscribing Topics
+
+* `~input` (`AnyMsg`)
+
+  Incoming topic to be relayed
+
+## Publishing Topics
+
+* `~output` (`AnyMsg`, same type as `~input`)
+
+  Outgoing topic to publish on
+
+## Services
+
+* `~request` (`std_srvs/Empty`)
+
+  Start topic relay
+
+* `~stop` (`std_srvs/Empty`)
+
+  Stop topic relay
+
+* `~request_duration` (`jsk_topic_tools/PassthroughDuration`)
+
+  Perform topic relay for a specified duration
+
+## Parameters
+
+* `~default_duration` (`double`, default: `10.0`)
+
+  Duration [s] to relay the topic.
+
+## Usage
+```
+# Terminal 1
+$ roslaunch jsk_topic_tools passthrough_sample.launch
+
+# Terminal 2
+$ rostopic pub /passthrough_sample/input std_msgs/String "data: 'hello'" -r10
+
+# Terminal 3
+$ rostopic echo /passthrough_sample/input
+
+# Terminal 4
+$ rostopic echo /passthrough_sample/output
+
+# Terminal 5
+$ rosservice call /passthrough_sample/request
+$ rosservice call /passthrough_sample/stop
+```

jsk_topic_tools/launch/passthrough_sample.launch

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+<launch>
+  <arg name="input" default="input" />
+  <arg name="output" default="output" />
+  <arg name="default_duration" default="10" />
+
+  <node pkg="jsk_topic_tools" name="passthrough_sample" type="passthrough">
+    <remap from="~input" to="$(arg input)" />
+    <remap from="~output" to="$(arg output)" />
+    <rosparam subst_value="true">
+      default_duration: $(arg default_duration)
+    </rosparam>
+  </node>
+
+</launch>