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.