Improve the docs telling needed environment variables with their values

[omatrot]

I came to your repo through a StackOverflow answer.
You're CMakeLists.txt file is using environment variables (OpenCV_LIBS in particular).
It would be nice to include the expected variables with their values.
It would save time to those people like me not very accustomed to OpenCV.
Thanks in advance.

[dmitry-zaitsev]

@omatrot if I remember correctly, you first need to install OpenCV using CMake.

Here is the guide for Linux. I believe steps 1, 2, 6 and 8 is all you need. After that, the variable will be defined.

[omatrot]

Which guide ?

[dmitry-zaitsev]

Oh, sorry. Here it is: https://docs.opencv.org/2.4/doc/tutorials/introduction/linux_install/linux_install.html?