docs: add MIT license, security policy, and README updates

Generic sample import instructions; expand CONTRIBUTING with upstream remote.

Co-authored-by: Cursor <cursoragent@cursor.com>

SECURITY.md

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+# Security policy
+
+## Supported versions
+
+Security fixes are applied on the default branch (`main`) when practical. Use the latest commit for deployments.
+
+## Reporting a vulnerability
+
+Please **do not** open a public GitHub issue for undisclosed security problems.
+
+Use **GitHub private vulnerability reporting** for this repository:
+
+[Submit a private security advisory](https://github.com/hansjone/netx/security/advisories/new)
+
+Include:
+
+- A short description of the impact
+- Steps to reproduce (or proof-of-concept) if you can share them safely
+- Affected API route, UI surface, or dependency (if known)
+
+We will treat reports as confidential and coordinate a fix and disclosure timeline with you when possible.