Added a README on variable shadowing

variable shadowing/README.md

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+# Variable Shadowing
+
+## Principle
+- Variable shadowing occurs when a variable declared within a certain scope (decision block, method, or inner class)
+has the same name as a variable declared in an outer scope.
+
+## Attack
+This depends a lot on the code of the contract itself. For instance, in the state_inherance.sol example, 
+it stops the owner to perform the self-destruction of the contract.
+
+## Mitigation
+The solidity compiler has [some checks](https://github.com/ethereum/solidity/issues/973) to emit warnings when 
+it detects this kind of issue, but [it has known examples](https://github.com/ethereum/solidity/issues/2563) where 
+it fails.