add IERC721Metadata

contracts/erc721/IERC721Metadata.sol

@@ -0,0 +1,19 @@
+pragma solidity ^0.6.1;
+
+/// @title ERC-721 Non-Fungible Token Standard, optional metadata extension
+/// @dev See https://eips.ethereum.org/EIPS/eip-721
+///  Note: the ERC-165 identifier for this interface is 0x5b5e139f.
+interface IERC721Metadata /* is ERC721 */ {
+    /// @notice A descriptive name for a collection of NFTs in this contract
+    function name() external view returns (string memory _name);
+
+    /// @notice An abbreviated name for NFTs in this contract
+    function symbol() external view returns (string memory _symbol);
+
+    /// @notice A distinct Uniform Resource Identifier (URI) for a given asset.
+    /// @dev Throws if `_tokenId` is not a valid NFT. URIs are defined in RFC
+    ///  3986. The URI may point to a JSON file that conforms to the "ERC721
+    ///  Metadata JSON Schema".
+    function tokenURI(uint256 _tokenId) external view returns (string memory);
+}
+