Audit available for the `xsalsa20poly1305`crate

[Erik1000]

I've found an audit of the XSalsa20Poly1305 crate by Cure53 funded by Threema. It seems like they didn't find anything particular problematic in the code base. I primarily opened this issue so you know that you can update the README to include the audit, since it currently says:

No security audits of this crate have ever been performed, and it has not been thoroughly assessed to ensure its operation is constant-time on common CPU architectures.

Link to the audit: https://cure53.de/pentest-report_rust-libs_2022.pdf 🚀