doc(readme): clarify the contributing guidelines (#306)

* Clarify the Contributing Guidelines in README

This PR updates the 'Contributing' section of the README to enhance clarity and make instructions straightforward for new contributors.

1. Formatted statements with bullet points as in other README files.
2. Updated the 'How to Contribute' link to a valid URL.
3. Added images of contributors.

These changes are intended to make the guide easier to follow and more engaging for new contributors.

* Update contribution link from markdown to website

- Changed the contribution guide link in the README.md from the markdown file to the corresponding section on the official website.
- This change ensures that the link remains stable and is not impacted by the frequent changes to Markdown files.

README.md

@@ -32,9 +32,11 @@ And here are links of other repositories:
 
 ## Contributing
 
-Welcome to contribute, please see [`How to Contribute`](https://github.com/apache/hugegraph/blob/master/CONTRIBUTING.md) for more information
+- Welcome to contribute to HugeGraph, please see [How to Contribute](https://hugegraph.apache.org/docs/contribution-guidelines/contribute/) for more information.
+- Note: It's recommended to use [GitHub Desktop](https://desktop.github.com/) to greatly simplify the PR and commit process.
+- Thank you to all the people who already contributed to HugeGraph!
 
-Note: It's recommended to use [GitHub Desktop](https://desktop.github.com/) to **greatly simplify** the PR and commit process.
+[![contributors graph](https://contrib.rocks/image?repo=apache/hugegraph-computer)](https://github.com/apache/incubator-hugegraph-computer/graphs/contributors)
 
 ## License