Add development guide

DEVELOPMENT.md

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+# Development
+
+Porcupine uses the standard [Go toolchain][golang] for development.
+
+[golang]: https://go.dev/
+
+## Testing
+
+You can run the tests with:
+
+```bash
+go test ./...
+```
+
+Some of the tests generate visualizations which must be manually inspected. To make the tests print the path to the generated HTML files, run the tests with the `-v` flag.
+
+## Formatting
+
+You can run the Go code formatter with:
+
+```bash
+go fmt ./...
+```
+
+Additionally, this project uses [Prettier] to format HTML, CSS, and JavaScript files. You can run Prettier (using [npx]) with:
+
+```bash
+npx prettier -w '**/*.{html,css,js}'
+```
+
+[Prettier]: https://prettier.io/
+[npx]: https://docs.npmjs.com/cli/v11/commands/npx
+
+## Static analysis
+
+You can run Go's built-in `vet` tool with:
+
+```bash
+go vet ./...
+```
+
+This project additionally uses the [staticcheck] tool. You can install it with:
+
+```bash
+go install honnef.co/go/tools/cmd/staticcheck@latest
+```
+
+You can run staticcheck with:
+
+```bash
+staticcheck -f stylish ./...
+```
+
+[staticcheck]: https://staticcheck.dev/
+
+## Continuous integration
+
+Testing and static analysis is [run in CI][ci-test].
+
+[ci-test]: .github/workflows/ci.yml