Add note about supply limit in ERC1155Supply

contracts/token/ERC1155/extensions/ERC1155Supply.sol

@@ -12,6 +12,9 @@ import "../ERC1155.sol";
  * clearly identified. Note: While a totalSupply of 1 might mean the
  * corresponding is an NFT, there is no guarantees that no other token with the
  * same id are not going to be minted.
+ *
+ * NOTE: This contract implies a global limit of 2**256 - 1 to the number of tokens
+ * that can be minted.
  */
 abstract contract ERC1155Supply is ERC1155 {
     mapping(uint256 => uint256) private _totalSupply;