feat: add section for Kerckhoff's principle

[caretak3r]

Pull Request Checklist

Please double check the items below!

• I have read the Contributor Guidelines.
• I have not directly copied text from another location (unless explicitly indicated as a quote) or violated copyright.
• I have linked to the original Law.
• I have quote the law (if possible) and the author's name (if possible).
• I am happy to have my changes merged, so that I appear as a contributor, but also the text altered if required to keep the language consistent in the project.

And don't forget:

• I can handle the table of contents, feel free to leave it out.
• Check to see if other laws should link back to the law you have added.
• Include your Twitter Handle if you want me to include you when tweeting this update!

[caretak3r]

twitter is @caretak8r

[caretak3r]

@dwmkerr Any updates here?