README: Note opencv-python dependency; don't mention GStreamer elements
README: Note opencv-python dependency; don't mention GStreamer elements

All image processing is now done in python code in stbt.py (0f0dc1a6),
and we no longer use any custom GStreamer elements (a35848f2).

Our image processing algorithm was always a custom one we developed
ourselves; the original GStreamer templatematch element merely called
the OpenCV "matchTemplate" function. So we no longer have any code
originating from the gstreamer-opencv project.

martynjarvis/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester