Document how to load multiple instances of the same "shared" worker

Author: codetheweb

docs/recipes/shared-workers.md

@@ -34,6 +34,28 @@ const shared = registerSharedWorker({
 
 Within a test process you can only register one worker for each `filename`. Filenames are compared as-is, without normalization. If you call `registerSharedWorker()` a second time, the same worker instance is returned.
 
+If for some reason you want to load the same file as multiple different workers, you can append a unique hash to the end of the filename:
+
+```js
+import crypto from 'crypto';
+import {registerSharedWorker} from 'ava/plugin';
+
+const key = Math.random() > 0.5 ? 'worker-a' : 'worker-b';
+
+const shared = registerSharedWorker<any>({
+  filename: new URL(
+    `file:${path.resolve(
+      __dirname,
+      'worker.js'
+    )}#${encodeURIComponent(key)}`
+  ),
+  initialData: {workerKey: key},
+  supportedProtocols: ['ava-4']
+});
+```
+
+This works because the `filename` parameter accepts [URL](https://nodejs.org/api/url.html) objects, meaning you could use a query component for the key instead if you wanted.
+
 You can supply a `teardown()` function which will be called after all tests have finished. If you call `registerSharedWorker()` multiple times then the `teardown()` function will be invoked for each registration, even though you only got one worker instance. The most recently registered `teardown()` function is called first, and so forth. `teardown()` functions execute sequentially.
 
 ```js