Add project index, templates, and docs

CONTRIBUTING.md

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+# Contributing to Minecraft Legacy Project Index
+
+Thanks for helping keep the project list up to date!
+
+## Adding a project via Pull Request
+
+1. **Fork** this repository.
+2. Open `projects.json` in any text editor.
+3. Add your entry to the `repos` array:
+
+```json
+{
+  "name": "YourName / YourProject",
+  "url": "https://github.com/YourName/YourProject",
+  "priority": 5
+}
+```
+
+4. **Priority** - use the next number after the last entry. Maintainers may adjust it.
+5. Make sure your JSON is valid - no trailing commas, all strings in double quotes.
+6. Commit with a message like: `Add YourProject to project list`
+7. Open a Pull Request and fill out the template.
+
+## Requesting a project (no code needed)
+
+Go to **Issues → New Issue → Project Request** and fill in the form. A maintainer will handle the rest.
+
+## Guidelines
+
+- One project per PR.
+- Project must be related to Minecraft Legacy / Console Edition.
+- No duplicate URLs.
+- Keep `projects.json` clean and formatted.
+
+## JSON validation
+
+Before submitting, paste your edited `projects.json` into [jsonlint.com](https://jsonlint.com) to check for errors.
+
+## Questions?
+
+Open an issue or reach out to the maintainers.