Add CONTRIBUTING.md with submission guidelines

CONTRIBUTING.md

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+# Contributing
+
+Thanks for helping grow this collection! Here's how to contribute.
+
+## Adding a system prompt
+
+1. Pick the right folder:
+   - `Anthropic/` — Claude models
+   - `OpenAI/` — ChatGPT, GPT, o-series, Codex
+   - `Google/` — Gemini models
+   - `xAI/` — Grok models
+   - `Perplexity/` — Perplexity models
+   - `Misc/` — everything else
+
+2. Create a `.md` file with a descriptive name (e.g. `gpt-5.4-thinking.md`, `gemini-3-flash.md`)
+
+3. Paste the raw system prompt as-is. Don't summarize or paraphrase — the full, unedited text is the point.
+
+4. Open a PR. That's it.
+
+## Requesting a prompt
+
+Don't have the prompt yourself? [Open an issue](https://github.com/asgeirtj/system_prompts_leaks/issues) with the model or product name and we'll try to track it down.
+
+## Updating an existing prompt
+
+Models get updated. If you have a newer version of an existing prompt, either update the file or add a new one with the version in the filename. Move the old one to the `old/` subfolder if applicable.