Add CONTRIBUTING.md pointing to the review guidelines

CONTRIBUTING.md

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+# Contributing to sed
+
+Hi! Welcome to uutils/sed, and thanks for wanting to contribute!
+
+This project follows the shared conventions of the [uutils](https://github.com/uutils)
+organization. Before opening a pull request, please read:
+
+- Our **[Review Guidelines](https://uutils.github.io/reviews/)** — what we expect
+  from a pull request and how reviews are carried out.
+- Our community's [CODE_OF_CONDUCT.md](./CODE_OF_CONDUCT.md), if present.
+
+Finally, feel free to join our [Discord](https://discord.gg/wQVJbvJ)!
+
+> [!WARNING]
+> uutils is original code and cannot contain any code from GNU or other
+> strongly-licensed (GPL/LGPL) implementations. We **cannot** accept changes
+> based on the GNU source code, and you **must not link** to it either. You may
+> look at permissively-licensed implementations (MIT/BSD) and read the GNU
+> *manuals* — never the GNU *source*.
+
+## In short
+
+- Discuss non-trivial changes in an issue **before** writing the code.
+- Keep pull requests **small, self-contained, and descriptively titled**
+  (e.g. `sed: fix ...`).
+- Make sure CI passes: tests are green, `rustfmt` is satisfied, and there are
+  no `clippy` warnings.
+- Add tests for new behavior; don't let coverage regress.
+- Write small, atomic commits annotated with the component you touched.
+
+See the [Review Guidelines](https://uutils.github.io/reviews/) for the full
+details.