Merge pull request #42 from sharronliu/master

README: Updated notes for opencv dependent

README.md

@@ -24,8 +24,7 @@ OA keeps integrating with various "state-of-the-art" algorithms.
   * [object_msgs](https://github.com/intel/object_msgs)
   * [ros_intel_movidius_ncs](https://github.com/intel/ros_intel_movidius_ncs) or [opencl_caffe](https://github.com/intel/ros_opencl_caffe)
 
-  NOTE: In older version of "ros-kinetic-opencv3" where OpenCV 3.2.0 was used, self-built opencv_tracking is needed. While this's no more necessary since OpenCV 3.3 integrated. Check the OpenCV version from "/opt/ros/kinetic/share/opencv3/package.xml"
-  * [opencv_tracking](https://github.com/opencv/opencv_contrib) tag 3.2.0
+  NOTE: OA depends on tracking feature from OpenCV (3.3 preferred, 3.2 minimum). The tracking feature is recently provided by ROS Kinetic package "ros-kinetic-opencv3" (where OpenCV **3.3.1** is integrated). However, if you're using an old version of ROS Kinetic (where OpenCV **3.2** is integrated), tracking feature is not provided. In such case you need self-build tracking from [opencv_contrib](https://github.com/opencv/opencv_contrib). It is important to keep opencv_contrib (self-built) and opencv (ROS Kinetic provided) in the same OpenCV version that can be checked from "/opt/ros/kinetic/share/opencv3/package.xml"
 
 ## build and test
   * to build