Update README.md

Add #6 to README

README.md

@@ -21,3 +21,57 @@ and select the specific node version.
 ## Demo Screen
 
 ![image](https://github.com/cennznet/spin2winApp/blob/master/resources/demo.gif)
+
+
+## Play with Contract
+
+#### 1. Build and deploy the contract
+You can follow this doc to Build and deploy the contract: [spin2win](https://cennznetdocs.com/CENNZNet/tutorials/spin2win.md).
+
+#### 2. Contract address and ABI file
+After you build and deploy the contract you will get two items to be used in the next step:
+- [Contract address](https://github.com/cennznet/spin2winApp/blob/master/src/services/network.js)
+- [Contract ABI  json file](https://github.com/cennznet/spin2winApp/blob/master/src/services/Spin2Win.json) 
+
+For `Contract ABI` of spin2win:
+
+```json
+"messages": [{
+    "name": "spin",
+    "selector": 2121348255,
+    "mutates": false,
+    "args": [{
+      "name": "player",
+      "type": "AccountId"
+    }],
+    "return_type": null
+  }]
+```
+
+You will see there is only one method in the contract, and the method name is `spin`, and only one parameter for `spin`, the type is `AccountId`.
+
+#### 3. Call contract
+
+#### [Construct a ContractABI instance](https://github.com/cennznet/spin2winApp/blob/master/src/services/spin2win.js):
+You can use this instance to call any messages in the contract ABI file with the correct args. The function will return a `payload` (hex value), this signature includes all the information of the message.
+
+#### [Create transaction](https://github.com/cennznet/spin2winApp/blob/master/src/services/sendReward.js):
+
+Creating [`api`](https://github.com/cennznet/spin2winApp/blob/7dd149e770ccd66bd4bced4e93d45e21cb6fcb35/src/services/cennz-lib.js#L25) and [`set signer`](https://github.com/cennznet/spin2winApp/blob/7dd149e770ccd66bd4bced4e93d45e21cb6fcb35/src/services/sendReward.js#L41).
+
+Calling `api.tx.contract.call` to get `tx` object:
+```javascript
+  const tx =  api.tx.contract.call(
+    contractAddress,
+    endowment, // deposit amount, can be 0: BigNumber
+    200000,    // gas fee
+    payload    // the payload when calling specific message
+  );
+```
+The last step: [`sign and send the tx`](https://github.com/cennznet/spin2winApp/blob/7dd149e770ccd66bd4bced4e93d45e21cb6fcb35/src/services/sendReward.js#L62). You can get the transaction status and all the event details inside the code and also check them on [CENNZnet UI](https://cennznet.js.org/cennznet-ui/#/explorer/query) with `block hash`.
+
+#### 4. Common issues
+
+-  The signer should have enough funds to sign a transaction. You can get tokens through [faucet](https://cennznet.js.org/faucet-ui/).
+-  The network(Localhost or Rimu) of deploying contract and calling contract should be the same one.
+- For spin2win contract, should make sure the contract has enough funds to allocate assets. You can also top up the contract, using contract address, through [faucet](https://cennznet.js.org/faucet-ui/).