doc: update doc

CONTRIBUTING.md

@@ -1,5 +1,35 @@
 # How to contribute to LibCmo21
 
-## Contributor Limit
+## Contributor Limitation
 
 This project does not allow contributions coming from employees who are or have been employees of Dassault.
+
+## Feature Limitation
+
+For the content of contribution, not all contribution will be accepted.
+Just like I said, we create **Minimalist** Virtools Environment.
+The basic RW functions is enough.
+We do not accept complex function implementations.
+There are 3 lists which indicate our accept guideline.
+
+### Wanted Features
+
+These features will be accepted as soon as possible.
+
+* The bug fix of any existing code.
+
+### Not Urgent Features
+
+These features are in plan, but not urge to merge.
+
+* The `CK_ID` remap system of Reader.
+* CK3dEntity hierarchy system.
+* Other CK classes implementations.
+* Non-Virtools 2.1 implementations.
+
+### Don't Care Features
+
+These features explicitly will not be merged.
+
+* Run Virtools file.
+* Plugin system.