add contribute steps doc

CONTRIBUTING.md

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+# Contributing to Defibus
+
+Welcome to Defibus! This document is a guideline about how to contribute to Defibus.
+If you find something incorrect or missing, please leave comments / suggestions.
+
+## Before you get started
+
+### Setting up your development environment
+
+You should have JDK installed in your operating system.
+
+## Contributing
+
+We are always very happy to have contributions, whether for typo fix, bug fix or big new features.
+Please do not ever hesitate to ask a question or send a pull request.
+
+We strongly value documentation and integration with other projects.
+We are very glad to accept improvements for these aspects.
+
+### GitHub workflow
+
+We use the `develop` branch as the development branch, which indicates that this is an unstable branch.
+
+Here are the workflow for contributors:
+
+1. Fork to your own
+2. Clone fork to local repository
+3. Create a new branch and work on it
+4. Keep your branch in sync
+5. Commit your changes (make sure your commit message concise)
+6. Push your commits to your forked repository
+7. Create a pull request
+
+Please follow [the pull request template](./.github/PULL_REQUEST_TEMPLATE.md).
+Please make sure the PR has a corresponding issue.
+
+After creating a PR, one or more reviewers will be assigned to the pull request.
+The reviewers will review the code.
+
+Before merging a PR, squash any fix review feedback, typo, merged, and rebased sorts of commits.
+The final commit message should be clear and concise.
+
+### Open an issue / PR
+
+We use [GitHub Issues](https://github.com/WeBankFinTech/Defibus/issues) and [Pull Requests](https://github.com/WeBankFinTech/Defibus/pulls) for trackers.
+
+If you find a typo in a document, find a bug in code, or want new features, or want to give suggestions,
+you can [open an issue on GitHub](https://github.com/WeBankFinTech/Defibus/issues/new) to report it.
+Please follow the guideline message in the issue template.
+
+If you want to contribute, please follow the [contribution workflow](#github-workflow) and create a new pull request.
+If your PR contains large changes, e.g. component refactor or new components, please write detailed documents
+about its design and usage.
+
+Note that a single pull request should not be too large. If heavy changes are required, it's better to separate the changes
+to a few individual PRs.
+
+### Code review
+
+All code should be well reviewed by one or more committers. Some principles:
+
+- Readability: Important code should be well-documented. Comply with our code style.
+- Elegance: New functions, classes or components should be well-designed.
+- Testability: Important code should be well-tested (high unit test coverage).
+
+## Community
+
+### Contact us