Update README

Author: janza

README.md

@@ -1,3 +1,10 @@
 [![Docker Automated buil](https://img.shields.io/docker/automated/jjanzic/docker-python3-opencv.svg)]()
 
 Docker image with python 3.6 and opencv 3.2
+
+Usage:
+
+    docker run -it docker-python3-opencv python
+    >>> import cv2
+
+Image tagged with :contrib contains docker image built with [contrib modules](https://github.com/opencv/opencv_contrib/)