chore: note about playground variants in contributing.md (#13741)

CONTRIBUTING.md

@@ -86,6 +86,8 @@ And re-run `pnpm install` to link the package.
 
 Each package under `playground/` contains a `__tests__` directory. The tests are run using [Vitest](https://vitest.dev/) + [Playwright](https://playwright.dev/) with custom integrations to make writing tests simple. The detailed setup is inside `vitest.config.e2e.js` and `playground/vitest*` files.
 
+Some playgrounds define variants to run the same app using different config setups. By convention, when running a test spec file in a nested folder in `__tests__`, the setup will try to use a config file named `vite.config-{folderName}.js` at the playground's root. You can see an example of variants in the [assets playground](https://github.com/vitejs/vite/tree/main/playground/assets).
+
 Before running the tests, make sure that [Vite has been built](#repo-setup). On Windows, you may want to [activate Developer Mode](https://docs.microsoft.com/en-us/windows/apps/get-started/enable-your-device-for-development) to resolve [issues with symlink creation for non-admins](https://github.com/vitejs/vite/issues/7390). Also, you may want to [set git `core.symlinks` to `true` to resolve issues with symlinks in git](https://github.com/vitejs/vite/issues/5242).
 
 Each integration test can be run under either dev server mode or build mode.