add qr back

README.md

@@ -149,7 +149,7 @@ English | [简体中文](README_ch.md)
 
 If you have any questions during the use of the model, you can join the official WeChat group to get more efficient questions and answers, and fully communicate with developers from all walks of life. We look forward to your joining.
 <div align="center">
-<img src="https://user-images.githubusercontent.com/23690325/147116815-d99c8b54-716f-4009-a6cd-87f2621d248f.jpg"  width = "200" height = "200" />
+<img src="./docs/imgs/joinus.PNG"  width = "200" height = "200" />
 </div>  
 please add WeChat above and send "Hub" to the robot, the robot will invite you to join the group automatically.