Docstring for `image_processing.py`

[tlentali]

In order to reach the highest pylint note, I added some doc-string in each beginning of each script. But I didn't fill it correctly.

https://github.com/tlentali/pycht/blob/31e3e5323f52eb556648e83d04be304bbf1cec37/pycht/image_processing.py#L1-L3

Sould be :

Provide the necessary code to generate a clustering, here a `kmeans` model, that will separate the different stencils by color.

It should serve as a quick reference to all the functions and arguments.

Docstring format :

• what are the most common python docstring formats
• styleguide

[clurton-betclic]

I'll have a look!