Carry the same licence and community files as the other repos

These are eight public repos that ship one product on one version, and
they had drifted: freemkv-engine carried no LICENSE at all (GitHub
reported its licence as none), two crates had no code of conduct, four
had no contributing guide, and none had a security policy — so there was
no private route to report a vulnerability in a disc-decryption tool.

SECURITY.md names GitHub Security Advisories on each repo rather than an
address, so there is nothing to keep in sync and no inbox to go stale.

CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# Contributing
+
+Thanks for your interest!
+
+- **Report a bug** — open an issue
+- **Fix a bug** — fork, branch, PR
+- **Suggest a feature** — open an issue first
+
+## Development
+
+```bash
+cargo build
+cargo test
+```
+
+## License
+
+MIT