CONTRIBUTING.md: correct how to sign the CLA (#1819)

* correct how to sign the CLA

CONTRIBUTING.md

@@ -36,8 +36,6 @@ Before submitting the code, we need to do some preparation:
    git config user.email "{email-address-of-github}" # like "jermy@apache.org"
    ```
 
-5. Sign the HugeGraph CLA: [https://cla-assistant.io/hugegraph/hugegraph](https://cla-assistant.io/hugegraph/hugegraph)
-
 Optional: You can use [GitHub desktop](https://desktop.github.com/) to greatly simplify the commit and update process.
 
 ## 2. Create an Issue on GitHub
@@ -110,6 +108,13 @@ Note that since GitHub requires submitting code through `username + token` (inst
 
 Go to the web page of GitHub fork repo, there would be a chance to create a Pull Request after pushing to a new branch, just click button "Compare & pull request" to do it. Then edit the description for proposed changes, which can just be copied from the commit message.
 
+Please sign the HugeGraph CLA when contributing code for the first time. You can sign the CLA by just posting a Pull Request Comment same as the below format:
+
+`I have read the CLA Document and I hereby sign the CLA`
+
+Note: please make sure the email address you used to submit the code is bound to the GitHub account. For how to bind the email address, please refer to https://github.com/settings/emails:
+<img width="1280" alt="image" src="https://user-images.githubusercontent.com/9625821/163522445-2a50a72a-dea2-434f-9868-3a0d40d0d037.png">
+
 ## 5. Code review
 
  Maintainers will start the code review after all the **automatic** checks are passed: