doc: add contributing guidance

CONTRIBUTING.md

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+# Contributing to Cloudreve
+
+Thank you for your interest in contributing to Cloudreve!
+
+The full contributing guide — including project structure, development environment
+setup, how to pick a task, how to submit a PR, and our AIGC (AI-generated code)
+policy — is maintained in our documentation site:
+
+**👉 https://docs.cloudreve.org/api/contributing**
+
+A few key points to keep in mind before you open a PR:
+
+- Cloudreve is dual-licensed. We only accept contributions to the **community
+  edition**, and all contributors must sign the
+  [CLA](https://cla-assistant.io/cloudreve/cloudreve) before a PR can be merged.
+- **PRs must be linked to an issue labeled with `Backlog`.** We do not accept
+  new features proposed directly through a PR. If you have a new idea, please
+  open an issue first and wait for it to be triaged and labeled.
+- **Each PR should correspond to a single change.** Please split large features
+  or refactors into multiple smaller PRs whenever possible.
+- If you use AI tools to help write code, please read the
+  [AIGC Guidelines](https://docs.cloudreve.org/api/contributing#aigc-guidelines)
+  first. We are not against AI, but we do not accept pure "vibe-coded" PRs.
+
+For discussion and support, join the `development` channel on our
+[Discord community](https://discord.com/channels/1343585183047094367/1343585679585579018).