Update README.md to include export step for installation on macOS

[kmans]

Adding in an important step to running GoCV on macOS. Without setting this environment variable, users running macOS may encounter an error similar to this one:

Package opencv4 was not found in the pkg-config search path.
Perhaps you should add the directory containing `opencv4.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opencv4' found
pkg-config: exit status 1

[deadprogram]

Normally you would only have this problem if OpenCV has been installed as a "keg-only" package. Perhaps you needed to run brew upgrade opencv due to having multiple OpenCV installs already?