Create robots.txt (#4439)

* Create robots.txt

* Update index.ts

assets/robots.txt

@@ -0,0 +1,4 @@
+user-agent: *
+allow: /
+
+# todo: sitemap

src/server/web/index.ts

@@ -79,6 +79,12 @@ router.get('/manifest.json', async ctx => {
 	});
 });
 
+router.get('/robots.txt', async ctx => {
+	await send(ctx as any, '/assets/robots.txt', {
+		root: client
+	});
+});
+
 //#endregion
 
 // Docs