Small improvements

[nagayev]

Add instruction how to install opencv
Fix part_4.py for modern opencv.

[wb-08]

In readme file you can write pip install -r requirements.txt instead of pip install opencv-python

[nagayev]

@wb-08 Yes, you are right, but there is only one requirement, so in my opinion the easiest way is to install opencv directly

[wb-08]

@nagayev , when you install opencv directly, then it installs the newest version of the library. And there is a chance that something will change in the new version again. And you can also please write a more specific version of opencv using ==

[nagayev]

@wb-08 Yes, you are right, but installing direct version of any heavy package like opencv makes your hard-drive unhappy.
I think the best way is installing opencv of certain range. E.g <5.0, because 5.0 is the next major release.