Create SECURITY.md

[Tajulka]

Pull request title

• I read contributing guideline
• I didn't find a similar pull request already open.
• 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.