Add OpenCV in the INSTALL instructions (#694)

INSTALL.md

@@ -24,7 +24,7 @@ conda activate maskrcnn_benchmark
 conda install ipython
 
 # maskrcnn_benchmark and coco api dependencies
-pip install ninja yacs cython matplotlib tqdm
+pip install ninja yacs cython matplotlib tqdm opencv-python
 
 # follow PyTorch installation in https://pytorch.org/get-started/locally/
 # we give the instructions for CUDA 9.0
@@ -39,7 +39,7 @@ cd cocoapi/PythonAPI
 python setup.py build_ext install
 
 # install apex
-cd ~github
+cd $INSTALL_DIR
 git clone https://github.com/NVIDIA/apex.git
 cd apex
 python setup.py install --cuda_ext --cpp_ext