Add some docs

[timholy]

I am starting to kick the tires here! Thanks for getting this working.

The first thing I noticed is that it would be nice to have a few items added to the README. For example:

• for potential users not coming from an OpenCV background, is there a recommended tutorial for getting started? I am guessing https://docs.opencv.org/master/d9/df8/tutorial_root.html
• where should potential contributors make changes? This package is essentially empty. It looks like https://github.com/archit120/JuliaOpenCVBindingGenerator is the place where all the actual code is defined, but it's called a "dummy package" in the README. It appears that https://github.com/archit120/JuliaOpenCVBindingGenerator/tree/master/src2/jl_cxx_files is the actual location of the source (or is it https://github.com/archit120/JuliaOpenCVBindingGenerator/tree/master/src2/autogen_jl?), but this is quite hard to find since this doesn't follow conventional Julia package organization. (Any chance we can move the actual Julia code here??)
• Since this doesn't quite have the conventional structure, how do I dev this package and test my changes?
• what version of OpenCV is this generated against? Perhaps there should be a way to query the OpenCV lib version? (If there is already such a method, can we document it?)
• The package version is 4.5.2, which I'm guessing you take from the library version of OpenCV itself. But suppose I want to make a breaking change to the Julia implementation of Mat. So to follow Semver now I need to release v5 of OpenCV.jl?

If you want to provide a few thoughts about these questions I am happy to submit a PR filling them out in greater detail in the README.