message: add tokenOfOwnerByIndexERC721 function in readme
diff: @@ -69,6 +69,14 @@ await matic.balanceOfERC721(
  options // transaction fields
  )
  +// get ERC721 token ID
  +await matic.tokenOfOwnerByIndexERC721(
  + from, // User address
  + tokenAddress, // Token address
  + index, // index of tokenId
  + options // transaction fields
  +)
  +
  // Deposit Ether into Matic chain
  await matic.depositEthers(
  options // transaction fields
  @@ -89,7 +97,8 @@ await matic.depositERC20Tokens(
  options // transaction fields
  )
  -// Deposit token into Matic chain.
  +// Deposit ERC721 token into Matic chain.(older ERC721 or some newer contracts will not support this.
  +// in that case, first call `approveERC721TokenForDeposit` and `depositERC721Tokens`)
  await matic.safeTransferFrom(
  token, // Token addres
  tokenId, // Token Id for deposit
  @@ -193,6 +202,7 @@ Please write to info@matic.network to request TEST tokens for development purpos
  - <a href="#initialize"><code>new Matic()</code></a>
  - <a href="#getMappedTokenAddress"><code>matic.<b>getMappedTokenAddress()</b></code></a>
  - <a href="#balanceOfERC721"><code>matic.<b>balanceOfERC721()</b></code></a>
  +- <a href="#tokenOfOwnerByIndexERC721"><code>matic.<b>tokenOfOwnerByIndexERC721()</b></code></a>
  - <a href="#depositEthers"><code>matic.<b>depositEther()</b></code></a>
  - <a href="#approveERC20TokensForDeposit"><code>matic.<b>approveERC20TokensForDeposit()</b></code></a>
  - <a href="#depositERC20Tokens"><code>matic.<b>depositERC20Tokens()</b></code></a>
  @@ -284,6 +294,33 @@ matic
  })
  ```
  +---
  +<a name="tokenOfOwnerByIndexERC721"></a>
  +
  +
  +#### matic.tokenOfOwnerByIndexERC721(address, token, index, options)
  +
  +get balance of ERC721 `token` for `address`.
  +
  +- `token` must be valid token address
  +- `address` must be valid user address
  +- `index` index of tokenId
  +
  +
  +This returns matic `balance`.
  +
  +Example:
  +
  +```js
  +matic
  + .tokenOfOwnerByIndexERC721("0xfeb14b...", "21", 0, {
  + from: "0xABc578455..."
  + })
  + .then(tokenID => {
  + console.log("Token ID", tokenID)
  + })
  +```
  +
  ---
  <a name="approveERC20TokensForDeposit"></a>
  