add Q&A section

[rainyl]

• [ ] used environment variables
  • [ ] OPENCV_DART_DISABLE_AUTO_BUILD
  • [ ] OPENCV_DART_ANDROID_ENABLED_ABI #211
  • [ ] OPENCV_DART_LIB_PATH
• [ ] libopencv_dart.so/.dll/.framework/.dylib not found
• [ ] symbol xxx not found https://github.com/rainyl/opencv_dart/issues/200#issuecomment-2278057007
• [ ] E/cv::error()(15188): OpenCV(4.10.0) Error: https://github.com/rainyl/opencv_dart/issues/179#issuecomment-2275672473
• [ ] instructions for cache cleaning up, manual setup
• [ ] video https://github.com/rainyl/opencv_dart/issues/159#issuecomment-2238065384