Add comments about requirements

requirements.txt

@@ -1,16 +1,32 @@
-# Do not add opencv here. Just like pytorch, user should install
+# PyTorch/TorchVision -------------------------
+# Do not add pytorch/torchvision here. For PyTorch and TorchVision
+# follow the official instructions at https://pytorch.org/get-started/locally/
+# to install PyTorch 1.7.1+ and torchvision 0.8.2+
+# torch>=1.7.1
+# torchvision>=0.8.2
+
+# base ----------------------------------------
+# Just like PyTorch, user should install
 # opencv themselves, preferrably by OS's package manager, or by
 # choosing the proper pypi package name at https://github.com/skvark/opencv-python
-Cython
+# opencv-python>=4.1.2
 matplotlib>=3.2.2
 numpy>=1.18.5
 pillow
 scipy>=1.4.1
 tqdm>=4.41.0
+
+# export --------------------------------------
 onnx>=1.8.0
+
+# plotting ------------------------------------
 ipython
-# pycocotools need python3.7 as minimal
-# pip install -U pycocotools>=2.0.2  # corresponds to https://github.com/ppwwyyxx/cocoapi
+tabulate
+
+# Lightning -----------------------------------
 pytorch_lightning==1.3.0rc1
 torchmetrics
-tabulate
+
+# extras --------------------------------------
+# pycocotools on PyPI needs python3.7 as minimal
+# pycocotools>=2.0.2  # corresponds to https://github.com/ppwwyyxx/cocoapi