Create CONTRIBUTING.md (sofastack#74)

CONTRIBUTING.md

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+## Contributing to SOFAJRaft
+
+SOFAJRaft is released under the Apache 2.0 license, and follows a very
+standard Github development process, using Github tracker for issues and
+merging pull requests into master. If you would like to contribute something, 
+or simply want to hack on the code this document should help you get started.
+
+### Sign the Contributor License Agreement
+Before we accept a non-trivial patch or pull request we will need you to
+sign the Contributor License Agreement. Signing the contributor’s agreement
+does not grant anyone commit rights to the main repository, but it does mean
+that we can accept your contributions, and you will get an author credit if
+we do. Active contributors might be asked to join the core team, and given
+the ability to merge pull requests.
+
+### Code Conventions
+None of these is essential for a pull request, but they will all help.
+
+1. we provided a [code formatter file](./tools/codestyle/formatter.xml), it 
+will formatting automatically your project when during process of building.
+
+2. Make sure all new `.java` files to have a simple Javadoc class comment
+with at least an `@author` tag identifying you, and preferably at least a
+paragraph on what the class is for.
+
+3. Add the ASF license header comment to all new `.java` files (copy from 
+existing files in the project)
+
+4. Add yourself as an `@author` to the `.java` files that you modify 
+substantially (more than cosmetic changes).
+
+5. Add some Javadocs.
+
+6. A few unit tests would help a lot as well — someone has to do it.
+
+7. When writing a commit message please follow [these conventions](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html),
+if you are fixing an existing issue please add Fixes gh-XXXX at the end of the commit message (where XXXX is the issue number).