add the memo

Author: 0xkenta

contract_upgradeability2.md

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+<!-- # ロジックコントラクトを変更する際の注意点
+you cannot change the order in which the contract state variables are declared, nor their type. 
+
+If you need to introduce a new variable, make sure you always do so at the end
+
+And if you remove a variable from the end of the contract, note that the storage will not be cleared. A subsequent update that adds a new variable will cause that variable to read the leftover value from the deleted one. -->