Update doc/nav_nomap.md, doc/nav_mapping.md, doc/nav_map.md files

doc/nav_map.md

+# Navigation in a map
+
+Use ROS move_base node to make the platform move in a built map.
+
+* Run the simulation of the model car in a world with some objects:
+    ```bash
+    roslaunch iri_model_car_gazebo sim.launch standalone:=false world:=objects
+    ```
+* Run the navigation algorithms. Here we are loading an already build map of the world `objects`.
+    ```bash
+    roslaunch iri_model_car_rosnav nav_map.launch map_path:=`rospack find iri_model_car_rosnav`/maps map_name:=objects
+    ```
+* Run rviz
+    ```bash
+    rviz -d `rospack find iri_model_car_rosnav`/rviz/model_car_nav_map.launch
+    ```
+* On Rviz 
+    * use the '2d Nav Goal' tool (shortcut 'g') and click-drag somewhere inside the map to send a goal
+    * The global planner will compute the path to reach the goal (blue line)
+    * The model car will start following this path until reachin the goal.

doc/nav_mapping.md

+# Creating a map
+
+Use SLAM/Gmapping to create an occupancy grid map.
+
+* Run the simulation of the model car in a world with some objects:
+    ```bash
+    roslaunch iri_model_car_gazebo sim.launch standalone:=false world:=objects
+    ```
+* Run the mapping algorithms:
+    ```bash
+    roslaunch iri_model_car_rosnav nav_mapping.launch
+    ```
+* Run rviz
+    ```bash
+    rviz -d `rospack find iri_model_car_rosnav`/rviz/model_car_nav_gmapping.launch
+    ```
+* Move the model car around with teleoperation node or sending navigation goals.
+    * `roslaunch iri_model_car_launch teleop_rqt.launch`
+    * On Rviz use the '2d Nav Goal' tool (shortcut 'g') and click-drag somewhere to send a goal
+
+* Map will be updated and showed in rviz as an image with:
+    * Free space as white cells
+    * Obstacles as black cells
+    * Unknown space as grey cells
+
+* When done building the map, save it somewhere giving it a name:
+    ```
+    roscd iri_model_car_rosnav/maps
+    rosrun map_server map_saver map:=model_car/map -f my_map_name
+    ```
+* Two files, my_map_name.pgm with the image and my_map_name.yaml with some parameters will be saved.

doc/nav_nomap.md

+# Navigation without a map
+
+Use ROS move_base node to make the platform move without a built map.
+
+* Run the simulation of the model car in a world with some objects:
+    ```bash
+    roslaunch iri_model_car_gazebo sim.launch world:=objects
+    ```
+* Run the navigation algorithms:
+    ```bash
+    roslaunch iri_model_car_rosnav nav_no_map.launch
+    ```
+* Run rviz
+    ```bash
+    rviz -d `rospack find iri_model_car_rosnav`/rviz/model_car_nav_no_map.launch
+    ```
+* On Rviz 
+    * use the '2d Nav Goal' tool (shortcut 'g') and click-drag somewhere inside the global costmap to send a goal
+    * The global planner will compute the path to reach the goal (blue line)
+    * The model car will start following the path until reaching the goal.