community: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Prospero.Eden Encore
+
+Thanks for helping improve Prospero.Eden Encore.
+
+Encore is a focused PS5 homebrew fork of ProsperoEden 1.000.040 whose primary validated target is
+**PS5 firmware 13.60**. Changes should preserve that focus unless a separate firmware has been
+validated on hardware.
+
+## Before opening an issue or pull request
+
+- Read the [README](README.md), [technical notes](docs/FORK_NOTES.md) and
+  [security policy](SECURITY.md).
+- Search existing issues and pull requests first.
+- Do not include console keys, firmware, copyrighted game data, account data or other private
+  material.
+- Security vulnerabilities must be reported privately according to [SECURITY.md](SECURITY.md),
+  not in a public issue.
+- FPKG entitlement/PPR and external kstuff/jailbreak problems are outside Encore's emulator scope
+  unless the report demonstrates a separate bug in Encore itself.
+
+## Development principles
+
+Prefer changes that are:
+
+1. **13.60-safe** — do not replace the known working elevation/runtime path without hardware proof.
+2. **Small and auditable** — avoid importing a large newer subsystem when a targeted fix is enough.
+3. **Recoverable** — preserve user settings/saves and provide an obvious way back from risky
+   settings.
+4. **Bounded** — caches, logs, allocations and retries should have explicit limits.
+5. **Reproducible** — dependencies must be pinned and release inputs verifiable.
+6. **Legal to redistribute** — keep third-party attribution and licenses accurate.
+
+## Building
+
+Linux is recommended.
+
+```bash
+make toolchain
+python3 -B tools/launcher/strings.py check
+make release
+```
+
+See [docs/BUILDING.md](docs/BUILDING.md) for the complete toolchain.
+
+## Pull requests
+
+A useful pull request should:
+
+- explain the problem and why the change belongs in Encore;
+- describe user-visible behavior;
+- identify security or compatibility trade-offs;
+- include tests or validation when practical;
+- keep documentation and translations in sync;
+- avoid unrelated refactors;
+- pass the release build and CodeQL.
+
+For PS5-specific behavior, state the tested firmware and whether validation was done on real
+hardware.
+
+## Third-party code
+
+Do not silently copy third-party code into the repository. Record its source, pinned revision and
+license in the appropriate dependency metadata and/or [THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md).
+
+## Style
+
+Match the surrounding code. Keep changes straightforward and reviewable. Prefer explicit checks over
+clever behavior in privileged, filesystem, parser and release code.