add more info about sync

src/backend.ts

@@ -1,6 +1,27 @@
 import EventEmitter from 'events';
 import { Chat } from './types';
 
+/*
+
+Sync and login requires a backend implementation.
+
+Example syncing:
+
+const customBackend = new MyCustomBackend();
+customBackend.register();
+
+In your custom backend, load saved chats from the server and call chatManager.loadChat(chat);
+
+chatManager.on('messages', async (messages: Message[]) => {
+    // send messages to server
+});
+
+chatManager.on('title', async (id: string, title: string) => {
+    // send updated chat title to server
+});
+
+*/
+
 export let backend: Backend | null = null;
 
 export class Backend extends EventEmitter {
@@ -13,21 +34,21 @@ export class Backend extends EventEmitter {
     }
 
     get isAuthenticated() {
+        // return whether the user is currently signed in
         return false;
     }
 
     async signIn(options?: any) {
+        // sign in the user
     }
 
     async shareChat(chat: Chat): Promise<string|null> {
+        // create a public share from the chat, and return the share's ID
         return null;
     }
     
     async getSharedChat(id: string): Promise<Chat|null> {
+        // load a publicly shared chat from its ID
         return null;
     }
-}
-
-export function getBackend() {
-    return backend;
 }