Update FEATURE_SET.md

doc/FEATURE_SET.md

@@ -123,6 +123,9 @@ The X2AP layer is based on **3GPP 36.423** v14.6.0 and implements the following
 
  - X2 Setup Request
  - X2 Setup Response 
+ - X2 Setup Failure
+ - Handover Request 
+ - Handover Request Acknowledge
 
 ## eNB Advanced Features ##