add YouJoin by @FreedomZZQ

Author: Joker

categories/social_network.md

@@ -150,7 +150,7 @@ App Name                   | Describe                  | Major Language
 ###Y
 App Name                   | Describe                  | Major Language             | Download 
 :------------------------: | :------------------------ | :------------------------: | :------------------------: 
- | | | 
+[YouJoin](https://github.com/FreedomZZQ/YouJoin-Android) | YouJoin Android Client, a cool app of connection, use Material Design. |  
 
 ###Z
 App Name                   | Describe                  | Major Language             | Download