Document mounting OPFS storage *after* the boot (#141)

Related to
Automattic/wordpress-playground-private#115

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Author: ashfame

packages/docs/site/docs/developers/06-apis/javascript-api/06-mount-data.md

@@ -17,3 +17,52 @@ window.showDirectoryPicker().then(function (directoryHandle) {
 	});
 });
 ```
+
+## Mount Browser's OPFS Storage
+
+You can mount OPFS storage available within the browser as well. Under the hood, we sync the memory filesystem to OPFS at the end of every PHP request served. It's advisable to delay mounting of OPFS after boot as shown below, so that WordPress installation doesn't trigger a sync of over 3000 files slowing down the boot process.
+
+```javascript
+const hasWordPressSiteInOPFS = false; // roll your logic to track this
+const blueprint = {
+	preferredVersions: {
+		php: '8.4',
+		wp: 'latest',
+	},
+	features: {
+		networking: true,
+	},
+	login: true,
+	steps: [], // add steps
+};
+
+try {
+	const mountDescriptor: MountDescriptor = {
+		device: {
+			type: 'opfs',
+			path: `my-unique-prefix/my-site`,
+		},
+		mountpoint: '/wordpress',
+		initialSyncDirection: hasWordPressSiteInOPFS ? 'opfs-to-memfs' : 'memfs-to-opfs',
+	};
+
+	const client = await startPlaygroundWeb( {
+		iframe: document.getElementById('wp'),
+		remoteUrl: 'https://playground.wordpress.net/remote.html',
+		blueprint: blueprint,
+		shouldInstallWordPress: ! hasWordPressSiteInOPFS,
+		mounts: hasWordPressSiteInOPFS ? [ mountDescriptor ] : [],
+	} );
+
+	if ( ! hasWordPressSiteInOPFS ) {
+		await client.mountOpfs( mountDescriptor );
+	}
+
+	await client.isReady();
+	return client;
+} catch ( error ) {
+	// handle error here
+}
+```
+
+For persistence guarantees, check [Storage quotes and eviction criterias](https://developer.mozilla.org/en-US/docs/Web/API/Storage_API/Storage_quotas_and_eviction_criteria).