Added tutorial for Gazebo joint controller plugin

examples/worlds/joint_controller.sdf

@@ -91,6 +91,7 @@
           </geometry>
           <material>
             <ambient>0.2 0.8 0.2 1</ambient>
+            <diffuse>0.8 0 0 1</diffuse>
           </material>
         </visual>
         <collision name="collision">
@@ -177,6 +178,7 @@
           </geometry>
           <material>
             <ambient>0.2 0.8 0.2 1</ambient>
+            <diffuse>0.8 0 0 1</diffuse>
           </material>
         </visual>
         <collision name="collision">

examples/worlds/joint_position_controller.sdf

@@ -85,6 +85,7 @@
           </geometry>
           <material>
             <ambient>0.2 0.8 0.2 1</ambient>
+            <diffuse>0.8 0 0 1</diffuse>
           </material>
         </visual>
         <collision name="collision">