Add OpenCV 4.x install instructions for Kinfu+

[lint-os]

In the Kinfu instructions, it is recommended to install OpenCV 4.x with the instructions at https://github.com/roboticslab-uc3m/installation-guides/blob/master/docs/install-opencv.md. However, this does not cover OpenCV 4.x or the build flag needed to get OpenCV to build with Kinfu. (more info here)

[lint-os]

I feel that rather than adding a whole new section of documentation, maybe it would be best in the Kinfu documentation include a link to the latest official build instructions (with a note about the -DOPENCV_ENABLE_NONFREE=TRUE
option) here.