Add helpers to use Kinetics 400 dataset

Author: RaivoKoot

Kinetics400/README.md

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+# Using Kineics 400
+This directory contains helpers to use the [Kinetics 400](https://github.com/cvdfoundation/kinetics-dataset) dataset with this 
+repository's VideoFrameDataset dataloader. Download it from [this URL](https://github.com/cvdfoundation/kinetics-dataset).
+
+### 1. Dataset Overview
+When you download the Kinetics 400 dataset, it comes in the following format:
+- An `.mp4` video file for every video
+- A `.csv` file for the training, validation, and testing annotations
+
+To use VideoFrameDataset with Kinetics 400, we need to
+1. Create a folder for every `.mp4` file that contains the RGB frames of that video.
+2. Turn each `.csv` file into an `annotations.txt` file, as described in the main README of this repository.
+
+### 2. Processing
+Doing (1) and (2) from above, is very easy if you use the python scripts provided in this directory.
+- For (1), make sure that all `.mp4` files (trainin, validation, and testing) are located in a single and the same
+directory. Run the script `videos_to_frames.py` and make sure that you set the file paths
+correctly inside of the script. This will probably take ~10 hours for Kinetics 400.
+- For (2), run the script `process_annotation_file.py` once for each annotation `.csv` and make sure that you
+set the file paths correctly inside of the script. You also must have completed step (1),
+before you are able to run this script.
+
+NOTE: The processed training, validation, and testing files that step (2) outputs, are uploaded here as well.
+You can directly use these and skip step (2). However, after completing step (1), you might have
+to run (2) yourself, to create these three annotation files yourself, in case there is some discrepancy between
+the way `videos_to_frames.py` extracts RGB frames on my machine compared to on yours (This is very likely. I 
+recommend running step 2 yourself).
+
+### 3. Done
+That's it! You should then have a folder on your disk `RGB` that contains all videos in individual RGB
+frames, and the three annotation files. This is all you need to use VideoFrameDataset and start training
+on Kinetics 400!