Readme document test runner config to avoid transpiling errors in dow…

…nstream testing (#54)

* Add Unit Test & stackblitz link in README (closes #48)

* bit more concise wording

Co-authored-by: Janosh Riebesell <[email protected]>

readme.md

@@ -348,6 +348,40 @@ You can alternatively style every part of this component with more fine-grained
 }
 ```
 
+## Downstream testing
+
+To test a Svelte component which imports `svelte-multiselect`, you need to configure your test runner to avoid [transpiling issues](https://github.com/EmilTholin/svelte-routing/issues/140#issuecomment-661682571).
+
+For Jest, exclude `svelte-multiselect` from `transformIgnorePatterns` in your `jest.config.json`:
+
+```json
+{
+  "transformIgnorePatterns": ["node_modules/?!(svelte-multiselect)"],
+  "transform": {
+    "^.+\\.[t|j]s?$": "esbuild-jest",
+    "^.+\\.svelte$": ["svelte-jester", { "preprocess": true }]
+  }
+}
+```
+
+For Vitest, include `svelte-multiselect` in `deps.inline`:
+
+```ts
+// vite.config.ts
+import { svelte } from '@sveltejs/vite-plugin-svelte'
+
+export default {
+  plugins: [svelte({ hot: !process.env.VITEST })],
+  test: {
+    deps: {
+      inline: [/svelte-multiselect/],
+    },
+  },
+}
+```
+
+Here's a [Stackblitz example](https://stackblitz.com/fork/github/davipon/test-svelte-multiselect?initialPath=__vitest__) that also uses [`'vitest-svelte-kit'`](https://github.com/nickbreaton/vitest-svelte-kit).
+
 ## Want to contribute?
 
 To submit a PR, clone the repo, install dependencies and start the dev server to try out your changes.