adding username

Author: Nursid

docs/user/2_onboarding.md

@@ -19,7 +19,12 @@ Here, you will have two options for signing up:
 ### Step 3: Authentication
 - **If you choose Google or Github**:
   - You will be redirected to Google's or Github authentication page for authentication.
-  - After successful authentication, a message will appear:  
+  - If you are a new user, you will be redirected to the username page.
+You need to create and add a username. After adding the username, you can proceed further.
+
+![username](../../static/img/username.png)
+
+- After successful authentication, a message will appear:  
     *"You have successfully signed in. You can close this Page, Now you can return to the app"*
   - Once the authentication process is completed, you can proceed to use the application.