Add virus scanning example to documentation

Hi, formidable is great for handling multipart uploads. One gap in the ecosystem is documentation around scanning uploaded files for malware before saving them.

I built pompelmi (https://github.com/pompelmi/pompelmi) — a zero-dependency ClamAV wrapper for Node.js that integrates cleanly with formidable's streaming API. Would a security section in the docs be welcome? Happy to open a PR.