Created Stereo mapping (markdown)

Stereo-mapping.md

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+Tutorial about mapping an environment using a stereo camera. The RTAB-Map standalone application cannot use directly a stereo camera as input. However, it can process saved stereo images in a database. It is what we will do here for this demonstration. 
+
+_To actually use a stereo camera with RTAB-Map, refer to [stereo configuration](http://wiki.ros.org/rtabmap_ros/Tutorials/SetupOnYourRobot#Stereo_B_.28recommended.29) of the rtabmap ROS package._ The database below was generated using this launch file [test_stereo_data_recorder.launch](https://github.com/introlab/rtabmap_ros/blob/master/launch/tests/test_stereo_data_recorder.launch) and a **calibrated** bumblebee camera: `roslaunch rtabmap_ros test_stereo_data_recorder.launch`
+
+***
+
+![stereo](https://raw.githubusercontent.com/wiki/introlab/rtabmap/doc/Tutorials/Stereo_indoor/stereo.png)
+
+## Demo
+
+ * Load this configuration file [stereo.ini](https://raw.githubusercontent.com/wiki/introlab/rtabmap/doc/Tutorials/Stereo_indoor/stereo.ini) in RTAB-Map (Menu Preferences->GUI Settings->"Load settings..." button).
+  * Here are the main parameters changed from the defaults:
+
+    | Parameter | Value |
+   | --- | --- |
+   | Odometry inlier distance | 0.1 m |
+   | Odometry minimum inliers | 10 |
+   | Odometry max depth | 10 m |
+   | GFTT maximum features | 500 |
+   | Source detection rate | 20 Hz |
+
+ * Download this database [stereo20Hz.db](stereo20Hz.db) containing stereo images at 20 Hz. 
+ * Select this database as source: "Menu Detection -> Select source -> Database..."
+ * Press start! When the mapping is finished, you should have something like the screenshot above.