ownable: add a notice on renounceOwnership (#937)
ownable: add a notice on renounceOwnership (#937)

* ownable: add a notice on renounceOwnership

* Apply the suggestion by @shrugs

OpenZeppelin/zep-solidity,OpenZeppelin/zeppelin-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zep-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zeppelin-solidity,OpenZeppelin/openzeppelin-contracts