License dedicated server packaging under GPLv3

CONTRIBUTING.md

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+# Contributing
+
+Coordinate substantial work with the NETfishing owners before beginning.
+
+By intentionally submitting material for inclusion, you agree to
+[`CONTRIBUTOR-TERMS.md`](CONTRIBUTOR-TERMS.md). Pull requests should include an
+affirmative statement such as:
+
+> I have read and agree to the repository’s contributor terms and have the
+> right to submit this contribution.
+
+Keep changes focused, preserve the server’s security and update boundaries,
+and do not commit game binaries, packages, credentials, deployment secrets,
+logs, persistent identities, or machine-specific configuration.
+
+Before review, run the relevant script syntax and packaging checks, followed
+by:
+
+```bash
+git diff --check
+git status --short
+```
+
+New source files should use `SPDX-License-Identifier: GPL-3.0-or-later` where
+their format permits comments.