Update CONTRIBUTING.md

[alex-ilyichov]

Pull request title

• [x ] I read contributing guideline
• [ x] I didn't find a similar pull request already open.
• [x ] My PR is related to Spleeter only, not a derivative product (such as Webapplication, or GUI provided by others)

Description

A few sentences describing the overall goals of the pull request's commits.

How this patch was tested

You tested it, right?

• [ ] I implemented unit test whicn ran successfully using poetry run pytest tests/
• [ ] Code has been formatted using poetry run black spleeter
• [ ] Imports has been formatted using `poetry run isort spleeter``

Documentation link and external references

Please provide any info that may help us better understand your code.