docs: add example for gasless transaction

docs/addtional-feature.md

@@ -25,7 +25,7 @@ Also notice that under `instant-finality-hack` mode, there might be some [compat
 
 The gas fee is preventing new users step into the web3 world. Users must learn to get the native token(CKB or ETH) before playing a blockchain game or exchanging tokens with a DEX. The gasless feature can provide a way for developers to sponsor transaction fees for users to give them a smooth experience.
 
-The gas feature is based on the ERC-4337 solution but way simpler. To use a such feature, users sign and send a special gasless transaction to call a specific smart contract named `Entrypoint`, then `Entrypoint` will call another smart contract named `Paymaster` deployed by developers to check if they are willing to pay the gas fee for this transaction. The special gasless transaction must satisfy the requirements:
+The gas feature is based on the [ERC-4337 solution](https://eips.ethereum.org/EIPS/eip-4337) but way simpler. To use a such feature, users sign and send a special gasless transaction to call a specific smart contract named `Entrypoint`, then `Entrypoint` will call another smart contract named `Paymaster` deployed by developers to check if they are willing to pay the gas fee for this transaction. The special gasless transaction must satisfy the requirements:
 
 - Must contain an `UserOperation` structure on the `tx.data` field, which contains the target contract and the paymaster address.
 - Must set `tx.gasPrice` to 0
@@ -34,7 +34,7 @@ The gas feature is based on the ERC-4337 solution but way simpler. To use a such
 ```sh
 struct UserOperation {
     address callContract;           # address of the target contract
-    bytes callData;                 # call data
+    bytes callData;                 # call data of the target contract
     uint256 callGasLimit;           # gas used to execute the call
     uint256 verificationGasLimit;   # gas used to verification
     uint256 maxFeePerGas;           # gas price
@@ -43,4 +43,40 @@ struct UserOperation {
 }
 ```
 
-More can be found [here](https://github.com/godwokenrises/godwoken/discussions/860#discussion-4568687)
+More specs can be found [here](https://github.com/godwokenrises/godwoken/discussions/860#discussion-4568687)
+
+### Example: How to use gasless transaction for your dapp
+
+Let's say we have the entrypoint contract at `0x9a11f47c0729fc56d9c44c059987d40703249569` and as a game developer, we want to pay the gas fee for some whitelist users, so we wrote a paymaster contract just like [this one](https://github.com/godwokenrises/account-abstraction/blob/gw-gasless/contracts/samples/GaslessDemoPaymaster.sol) and deployed it at `0x6b019795aa36dd19eb4a4d76f3b9a40239b7c19f`.
+
+dapp frontend using ethers.js
+
+```ts
+      // define UserOp
+      const userOp: UserOperationStruct = {
+          callContract: realGameContract.address,
+          callData: realGameContractCallData,
+          callGasLimit: gasToExecuteRealGameContractCallData,
+          verificationGasLimit: gasToVerifyPaymaster,
+          maxFeePerGas: gasPrice,
+          maxPriorityFeePerGas: gasPrice,
+          paymasterAndData: "0x6b019795aa36dd19eb4a4d76f3b9a40239b7c19f" 
+      }
+
+      // construct and send gasless transaction
+      const abiCoder = new ethers.utils.AbiCoder();
+      const payload = abiCoder.encode(["tuple(address callContract, bytes callData, uint256 callGasLimit, uint256 verificationGasLimit, uint256 maxFeePerGas, uint256 maxPriorityFeePerGas, bytes paymasterAndData) UserOperation"], [userOp])
+
+      const gaslessTx = {
+        from: whitelistUser.address,
+        to: '0x9a11f47c0729fc56d9c44c059987d40703249569',
+        data: payload,
+        gasPrice: 0,
+        gasLimit: 1000000,
+        value: 0,
+      }
+
+      const signer = new ethers.Wallet("private key");
+      const tx = await signer.sendTransaction(gaslessTx);
+      await tx.wait()
+```