Security: add virus scanning example to README

Hi, I wanted to suggest adding a security section to the README
covering virus scanning of uploaded files.

Most developers using multer don't realize their upload endpoints
accept malware without any content inspection. A brief example
using memoryStorage + scanBuffer would help a lot.

I built pompelmi (https://github.com/pompelmi/pompelmi) specifically
for this use case — zero dependencies, typed verdicts, works directly
with multer's memoryStorage. Happy to contribute a PR adding the
example to the docs if that would be welcome.