Add Terms and Conditions link to MerkleDrop contract

[weilbith]

• Add link as comment into the header of the Solidity file of the contract like a license.
• We must make sure that this remains in the code when verifying the contract on Etherscan (don't delete all comments e.g. by a code flattener)

[weilbith]

This has been already solved in trustlines-network/project#779 I will exchange it as the actual requirement for the story of trustlines-network/project#756