message: Update README.md
  Added Docker example
diff: @@ -45,6 +45,40 @@ When launching `rtabmap_ros`'s nodes, if you have the error `error while loading
  $ export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/opt/ros/kinetic/lib/x86_64-linux-gnu
  ```
  +### Docker
  +
  +* Install rtabmap inside an official ros docker image (example with melodic):
  + ```bash
  + $ docker pull ros:melodic-perception
  + $ docker run -it --rm --name=ros-melodic-rtabmap ros:melodic-perception
  +
  + # Indide the container:
  + $ apt update && apt install -y ros-melodic-rtabmap-ros && echo "source /opt/ros/melodic/setup.bash" >> ~/.bashrc
  +
  + # Before exiting the container, commit from another terminal on host:
  + $ docker commit ros-melodic-rtabmap ros:melodic-rtabmap
  + # exit container...
  + ```
  + * Note that we could build rtabmap from source in the container if we want latest version, following instructions below. Just make sure to do "`echo "source /catkin_ws/devel/setup.bash" >> ~/.bashrc`" in the container so that the following example can still work as is.
  +
  +* The following example show to launch a camera on host computer and run our pre-built rtabmap container. All examples from [RGB-D tutorial](http://wiki.ros.org/rtabmap_ros/Tutorials/HandHeldMapping) and [stereo tutorial](http://wiki.ros.org/rtabmap_ros/Tutorials/StereoHandHeldMapping) should work using rtabmap from the container instead. Launch camera on host computer (set ROS_IP as the IP used for docker):
  + ```bash
  + $ export ROS_IP=172.17.0.1 && roslaunch openni2_launch openni2.launch depth_registration:=true
  + # In another terminal, launch rtabmapviz or RVIZ. We do visualization
  + # on host computer to avoid GPU problems with the container:
  + $ export ROS_NAMESPACE=rtabmap && rosrun rtabmap_ros rtabmapviz _frame_id:=camera_link
  + ```
  +
  +* Launch rtabmap from inside the container, saving the database on host `~/.ros/rtabmap.db`:
  + ```bash
  + $ docker run -it --rm \
  + --env ROS_MASTER_URI=http://172.17.0.1:11311 --env ROS_IP=172.17.0.2 \
  + -v ~/.ros:/root \
  + ros:melodic-rtabmap \
  + roslaunch rtabmap_ros rtabmap.launch rtabmapviz:=false database_path:=/root/rtabmap.db rtabmap_args:="--delete_db_on_start"
  + ```
  +
  +
  ## Build from source
  This section shows how to install RTAB-Map ros-pkg on **ROS Hydro/Indigo/Jade/Kinetic/Lunar** (Catkin build). RTAB-Map works only with the PCL >=1.7, which is the default version installed with ROS Hydro/Indigo/Jade/Kinetic/Lunar (**Fuerte and Groovy are not supported**).
  