docs: update README.md

I'm looking for long-term contributors/partners to the leetcode repo! Send me PRs if you're interested! See the following:
- Fork the repo(https://github.com/doocs/leetcode) to your own GitHub account and then clone it to your local machine.
- Make some changes to your leetcode repository, then add, commit and push it to your remote GitHub repository.
- Submit a pull request with your changes!
- See https://github.com/doocs/leetcode/blob/master/.github/CONTRIBUTING.md for more details.

README.md

@@ -3,6 +3,7 @@
 [![original](https://badgen.net/badge/original/%E4%B8%AD%E5%8D%8E%E7%9F%B3%E6%9D%89/orange)](https://github.com/doocs/advanced-java)
 [![open-source-organization](https://badgen.net/badge/organization/join%20us/pink)](https://github.com/doocs/intro)
 [![reading](https://badgen.net/badge/books/read%20together/cyan)](https://github.com/doocs/technical-books)
+[![leetcode](https://badgen.net/badge/leetcode/coding%20together/cyan)](https://github.com/doocs/leetcode)
 [![stars](https://badgen.net/github/stars/doocs/advanced-java)](https://github.com/doocs/advanced-java/stargazers)
 [![forks](https://badgen.net/github/forks/doocs/advanced-java)](https://github.com/doocs/advanced-java/network/members)
 [![help-wanted](https://badgen.net/github/label-issues/doocs/advanced-java/help%20wanted/open)](https://github.com/doocs/advanced-java/labels/help%20wanted)