docs: add security policy

SECURITY.md

@@ -0,0 +1,20 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 5.x     | :white_check_mark: |
+| < 5.x   | :x:                |
+
+## Reporting a Vulnerability
+
+- **Critical:** report privately via the [Security](https://github.com/hacdias/webdav/security) page.
+- **Non-critical:** open a public issue so the community can help; it'll be labeled it as a security issue.
+
+Please include, where possible:
+
+- The commit the issue was found at
+- A plaintext proof of concept (no binaries)
+- Steps to reproduce
+- Recommended remediation, if any