Added Custom World Simulation section.

gazebo_basics.md

@@ -1,8 +1,41 @@
 # Spawning Stretch in Simulation (Gazebo)
-<!-- ![image](images/stretch_gazebo_empty_world.png) -->
-<img src="images/stretch_gazebo_empty_world.png" width="500" align="center">
 
-Run the following command to spawn Stretch in gazebo's default empty world.
+### Empty World Simulation
+To spawn the Stretch in gazebo's default empty world run the following command in your terminal.
 ```
 roslaunch stretch_gazebo gazebo.launch
 ```
+This will bringup the robot in the gazebo simulation similar to the image shown below.
+
+<!-- <img src="images/stretch_gazebo_empty_world.png" width="500" align="center"> -->
+![image](images/stretch_gazebo_empty_world.png)
+
+### Custom World Simulation
+With a modification of the gazebo launch file, you can spawn the Stretch robot in a custom world. In this instance, we will spawn Stretch in one of gazebo's world files named *willowgarage.world*.
+
+To preserve the original gazebo.launch file, we will copy the file and rename it to the following.
+
+```
+cd catkin_ws/src/stretch_ros/stretch_gazebo/launch/
+cp gazebo.launch stretch_willowgarage_world.launch
+```
+
+Modify the value for the world name argument like the following
+```
+<arg name="world_name" value="worlds/willowgarage.world" />
+```
+Use the images below for further reference.
+
+**BEFORE**
+![image](images/before_launch_file_edit.png)
+
+**AFTER**
+![image](images/after_launch_file_edit.png)
+
+Make sure to save the file. Also, you can make the modifications to the file using your preferred text editor. Now you can run the new launch file with a Stretch robot in the willowgarage.world.
+
+```
+roslaunch stretch_gazebo stretch_willowgarage_world.launch
+```
+
+![image](images/stretch_willowgarage_world.png)