Add OpenCV in the INSTALL instructions

[rodrigoberriel]

Minor stuff on the INSTALL instructions:

1. After b4d5465, OpenCV is now required. We can either perform a check (maybe the user is using only Faster R-CNN) or just add opencv-python in the INSTALL instructions. This PR goes with the second option;

2. This PR also fixes the ~github used during the apex install. The convention was adopted after the beginning of the mixed-precision PR.

If you do a fresh install and try to run the train command on the README, you get this error:

Traceback (most recent call last):
  File "tools/train_net.py", line 15, in <module>
    from maskrcnn_benchmark.data import make_data_loader
  File "/home/berriel/projects/maskrcnn-benchmark-fork/maskrcnn_benchmark/data/__init__.py", line 2, in <module>
    from .build import make_data_loader
  File "/home/berriel/projects/maskrcnn-benchmark-fork/maskrcnn_benchmark/data/build.py", line 10, in <module>
    from . import datasets as D
  File "/home/berriel/projects/maskrcnn-benchmark-fork/maskrcnn_benchmark/data/datasets/__init__.py", line 2, in <module>
    from .coco import COCODataset
  File "/home/berriel/projects/maskrcnn-benchmark-fork/maskrcnn_benchmark/data/datasets/coco.py", line 6, in <module>
    from maskrcnn_benchmark.structures.segmentation_mask import SegmentationMask
  File "/home/berriel/projects/maskrcnn-benchmark-fork/maskrcnn_benchmark/structures/segmentation_mask.py", line 1, in <module>
    import cv2
ImportError: No module named 'cv2'

[fmassa]

Thanks!