Add vendor (steam)

vendor/github.com/Philipp15b/go-steam/notifications_events.go

@@ -0,0 +1,9 @@
+package steam
+
+// This event is emitted for every CMsgClientUserNotifications message and likewise only used for
+// trade offers. Unlike the the above it is also emitted when the count of a type that was tracked
+// before by this Notifications instance reaches zero.
+type NotificationEvent struct {
+	Type  NotificationType
+	Count uint
+}