Add notes for payable function calls.

Author: MoMannn

README.md

@@ -5,6 +5,7 @@
 This is the complete reference implementation of the [ERC-721](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md) non-fungible token standard for the Ethereum and Wanchain blockchains. This is an open-source project, complete with [Hardhat](https://hardhat.org/) testing.
 
 The purpose of this implementation is to provide a good starting point for anyone who wants to use and develop non-fungible tokens on the Ethereum and Wanchain blockchains. Instead of re-implementing the ERC-721 yourself you can use this code which has gone through multiple audits and we hope it will be extensively used by the community in the future.
+Note that this implementation is more restrictive then the ERC-721 standard since it does not support `payable` function calls out of the box. You are however free to add this yourself.
 
 If you are looking for a more feature-rich and advanced ERC721 implementation, then check out the [0xcert Framework](https://github.com/0xcert/framework).
 

src/contracts/tokens/erc721.sol

@@ -48,7 +48,8 @@ interface ERC721
    * function checks if `_to` is a smart contract (code size > 0). If so, it calls
    * `onERC721Received` on `_to` and throws if the return value is not
    * `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`.
-   * @dev Transfers the ownership of an NFT from one address to another address.
+   * @dev Transfers the ownership of an NFT from one address to another address. This function can
+   * be changed to payable.
    * @param _from The current owner of the NFT.
    * @param _to The new owner.
    * @param _tokenId The NFT to transfer.
@@ -65,7 +66,8 @@ interface ERC721
   /**
    * @notice This works identically to the other function with an extra data parameter, except this
    * function just sets data to ""
-   * @dev Transfers the ownership of an NFT from one address to another address.
+   * @dev Transfers the ownership of an NFT from one address to another address. This function can
+   * be changed to payable.
    * @param _from The current owner of the NFT.
    * @param _to The new owner.
    * @param _tokenId The NFT to transfer.
@@ -82,7 +84,7 @@ interface ERC721
    * they may be permanently lost.
    * @dev Throws unless `msg.sender` is the current owner, an authorized operator, or the approved
    * address for this NFT. Throws if `_from` is not the current owner. Throws if `_to` is the zero
-   * address. Throws if `_tokenId` is not a valid NFT.
+   * address. Throws if `_tokenId` is not a valid NFT.  This function can be changed to payable.
    * @param _from The current owner of the NFT.
    * @param _to The new owner.
    * @param _tokenId The NFT to transfer.
@@ -98,7 +100,7 @@ interface ERC721
    * @notice The zero address indicates there is no approved address. Throws unless `msg.sender` is
    * the current NFT owner, or an authorized operator of the current owner.
    * @param _approved The new approved NFT controller.
-   * @dev Set or reaffirm the approved address for an NFT.
+   * @dev Set or reaffirm the approved address for an NFT. This function can be changed to payable.
    * @param _tokenId The NFT to approve.
    */
   function approve(