docs(contributing): add black tool requirement for autogen.sh (#434)

Without black being installed, I get the error: black is not installed

CONTRIBUTING.md

@@ -30,16 +30,17 @@ ffmpeg -i my_big_image.jpg -vf scale=w=256:h=256:force_original_aspect_ratio=dec
 
 Then just run the `./autogen.sh` to generate the added or updated node classes. (cf. [DEVELOPMENT][DEVELOPMENT.md])
 
-> IMPORTANT NOTE: To run `autogen.sh`, you need [round][round] and
+> IMPORTANT NOTE: To run `autogen.sh`, you need [round][round], [black][black] and
 > [inkscape][inkscape] command lines that are used for cleaning the image
-> resource filenames.
+> resource filenames and formatting the generated python code.
 >
 > macOS users can download the inkscape via Homebrew.
 >
 > Or you should use the docker image.
 
 [DEVELOPMENT.md]: ./DEVELOPMENT.md
 [round]: https://github.com/mingrammer/round
+[black]: https://pypi.org/project/black
 [inkscape]: https://inkscape.org/ko/release
 
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