Missing usage example for live stream/camera

[Breakthrough]

As identified in #151, there is no example in the API documentation for how to use a callback, e.g. for processing a live video stream. An example should clearly document and demonstrate this use case, e.g.:

from scenedetect import VideoStreamCv2, SceneManager, ContentDetector
scene_manager = SceneManager()
cam = VideoStreamCv2(0)
scene_manager.add_detector(ContentDetector())
scene_manager.detect_scenes(video=cam, callback=lambda frame: print("a new scene: ", len(frame)))

Also note that the callback API is still unstable, so if you are using this in production you should pin your scenedetect package to < 0.7.

---

This arose from the following comment:

A usage example:

cam = cv2.VideoCapture(0)
scene_manager.add_detector(ContentDetector())
scene_manager.detect_scenes(frame_source=cam, callback=lambda frame: print("a new scene: ", len(frame)))

when i tried this on a live video input, it throws error,

    scene_manager.detect_scenes(frame_source=cam, callback=lambda frame: print("a new scene: ", len(frame)))
  File "/usr/local/lib/python3.6/dist-packages/scenedetect/scene_manager.py", line 752, in detect_scenes
    self._base_timecode = video.base_timecode
AttributeError: 'cv2.VideoCapture' object has no attribute 'base_timecode'
```
any idea why is this happening?

Originally posted by @research-boy in #151 (comment)

[Breakthrough]

Can see updated example in the v0.6.1 branch.