[visualization_tutorial] Add trajectory slider (#79)

Add trajectory slider (moveit/moveit#491).

Author: 130s

doc/ros_visualization/pr2_moveit_pr491.png

@@ -103,7 +103,7 @@ Step 3: Interact with the PR2
    :width: 500px
 
 Moving into collision
-----------------------
++++++++++++++++++++++
 
 Note what happens when you try to move one of the arms into collision
 with the other. The two links that are in collision will turn red.
@@ -123,7 +123,7 @@ checkbox.
    :width: 300px
 
 Moving out of reachable workspace
------------------------------------
++++++++++++++++++++++++++++++++++
 
 Note also what happens when you try to move an end-effector out of its
 reachable workspace (sometimes the access denied sign will not
@@ -152,6 +152,24 @@ Step 4: Use Motion Planning with the PR2
 .. image:: rviz_plugin_planned_path.png
    :width: 700px
 
+Introspecting trajectory waypoints
+++++++++++++++++++++++++++++++++++
+
+*Available since: Indigo 0.7.10, Kinetic and Lunar 0.9.7*
+
+You can visually introspect trajectory point by point on RViz.
+
+* From "`Panel`" menu, enable "`MotionPlanning - Slider`" that is disabled by default. You'll see a new Slider panel on RViz.
+
+* Set your goal pose, then run `Plan`.
+
+* Play with the "`Slider`" panel, e.g. move the slider, push "`Play`" button.
+
+NOTE: Once you placed your EEF to a new goal, be sure to run `Plan` before running `Play` -- otherwise you'll see the waypoints for the previous goal if available.
+
+.. image:: pr2_moveit_pr491.png
+   :width: 700px
+
 What's Next
 -------------