Docs: a brief introduction to OpenCV

[gromag]

For those unfamiliar with OpenCV (particularly OpenCV Python), I have added a brief introduction to provide an overview of what OpenCV is about. The current README jumps straight into installation instructions without providing a clear description of the repository. While experienced OpenCV developers may not require an introduction, I believe it's important to be inclusive and help new developers understand what OpenCV is about.

Additionally, I suggest adding a brief introduction to the main OpenCV repository to improve accessibility for all users.