Add contribution checklist

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+# Contributing
+
+Checklist for changes and releases.
+
+## 1. Type checker
+
+Run the type checker:
+
+```
+npm run types:check
+```
+
+All reasonable type issues (i.e., those not resulting from limitations of `tsc`) should be fixed.
+
+## 2. Tests
+
+Run tests:
+
+```
+npm run test
+npm run test:large
+```
+
+All tests should pass.
+
+Check code coverage:
+
+```
+npm run test:coverage
+```
+
+Coverage should be maintained at 100%.
+
+## 3. Benchmarks
+
+Run benchmarks before and after changes to check for performance regressions:
+
+```
+npm run bench
+```
+
+Performance regressions should be fixed or explained.
+
+## 4. Debugging code
+
+Search for and delete code that was written for debugging purposes:
+- `debugger`
+- `only: true` (in tests)
+- `console`
+- ...etc.
+
+No debugging code should be committed.
+
+## 5. (Release only) Dev package
+
+Create a dev package to test the latest changes:
+
+```
+npm run pack:dev
+```
+
+A dev package is identical to a production package created by `npm pack`, except:
+- `version` is set to `0.0.0-DEV-[timestamp]`.
+- `private` is set to `true` to prevent accidental publishing.
+
+Move the `.tgz` package to a new Node.js project and then install the package:
+
+```
+npm i -D ./ardislu-csvio-0.0.0-DEV-NNNNNNNNNNNNN.tgz
+```
+
+Manually review for issues (i.e., try integrating the library in the Node.js project).
+
+## 6. (Release only) `package.json` and `package-lock.json`
+
+Update the `version` key in `package.json`.
+
+Bump dependencies in:
+- `package.json`
+- `release.yml`
+- `.node-version`
+
+Do `npm install` to update `package-lock.json`.
+
+## 7. (Release only) Release tag
+
+Cut a new release by adding a new annotated tag.
+
+Use the "v" prefix for versions (i.e., the correct tag is **"v1.0.0"**, NOT "1.0.0").
+
+Put release notes in the tag message:
+- The first line is the subject line and should always be "Release $VERSION".
+- The body should be a human-friendly summary of important changes (see [`esbuild`](https://github.com/evanw/esbuild/releases) release notes for reference).
+- Use Markdown for the body.
+
+Example:
+
+```
+$VERSION="v1.0.0"
+git tag -a $VERSION -m "Release $VERSION
+
+Example release note description:
+- Example point 1
+- Example point 2
+- Example point 3
+
+Example sentence with [a link](https://example.com)."
+```
+
+To check the release notes for a version:
+
+```
+$VERSION="v1.0.0"
+git for-each-ref --format="%(contents:subject)%0a%0a%(contents:body)" "refs/tags/$VERSION"
+```
+
+The tags can also be viewed in [GitHub](https://github.com/ardislu/csvio/tags) or [Codeberg](https://codeberg.org/ardislu/csvio/tags).