Document npm tasks for bundler integrations

docs/contributing/tasks.md

@@ -99,3 +99,13 @@ This task will:
 ## Review app only
 
 After building the project with `npm run build` the Express.js review app can be started with `npm start --workspace @govuk-frontend/review`. This prevents the Gulp tasks triggered by `npm start` from running.
+
+## Bundler integration
+
+After building the project with `npm run build`, you can verify that the `govuk-frontend` package will be consumed correctly by mainstream bundlers with `npm run <BUNDLER_NAME> --workspace @govuk-frontend/bundler-integrations` (where bundler name is one of `rollup`, `webpack` or `vite`).
+
+This will use the specified bundler to compile `.github/workflows/bundler-integrations/src/index.mjs`. That file only imports one components, which will let us verify that [tree shaking] works as intended. The build output (in `.github/workflows/bundler-integrations/dist/<BUNDLER_NAME>/`) should not contain the code of other components.
+
+You can also run `npm run build:all --workspace @govuk-frontend/bundler-integrations` to run all three bundlers in one go.
+
+[tree shaking]: https://developer.mozilla.org/en-US/docs/Glossary/Tree_shaking