Write a readme for mission part

GameServer/Game/Mission/README.md

@@ -0,0 +1,47 @@
+# Mission Part
+## Menu
+- [MissionManager](#missionmanager)
+- [FinishTypeHandler](#finishtypehandler)
+- [FinishActionHandler](#finishactionhandler)
+
+## MissionManager
+- `MissionManager.cs`: Used to manage the player's mission data
+
+### Method
+- (need to write)
+
+
+## FinishTypeHandler
+
+Class Name Style: **MissionHandler<Type>**  
+Example:
+```
+[MissionFinishType(MissionFinishTypeEnum.<Type>)]
+public class MissionHandler<Type> : MissionFinishTypeHandler
+{
+    public override void Init(PlayerInstance player, SubMissionInfo info, object? arg)
+    {
+        // your code
+    } 
+
+    public override void HandleFinishType(PlayerInstance player, SubMissionInfo info, object? arg)
+    {
+        // your code
+    }
+}
+```
+
+## FinishActionHandler
+
+Class Name Style: **MissionHandler<Type>**  
+Example:
+```
+[MissionFinishAction(FinishActionTypeEnum.<Type>)]
+public class MissionHandler<Type> : MissionFinishActionHandler
+{
+    public override void OnHandle(List<int> Params, List<string> ParamString, PlayerInstance Player)
+    {
+        // your code
+    }
+}
+```