CONTRIBUTING.md file for github

CONTRIBUTING.md

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+Contributions to the seL4 kernel repository are welcome!
+
+
+## CLA
+
+Please note that GD and NICTA require a contributor license agreement (CLA)
+to be signed for all external contributions. Please see
+https://seL4.systems/Contributing for details (it is a fairly short and
+standard CLA).
+
+For the `seL4/seL4` repository, we can review pull requests directly on
+github if we have a signed CLA on file.
+
+If you have only small trivial changes such as style, typos, comments, or
+white space and don't want to sign a CLA for that, please file an issue in
+the github issue tracker, we'll usually be happy to do the change ourselves
+and attribute your idea by linking to the github issue in the change set
+comment.
+
+
+## Kernel Development Process
+
+Please read https://seL4.systems/Contributing
+
+
+## Build/Test
+
+Generally, any contributions should pass the tests in the project
+https://github.com/seL4/sel4test. If new features or platforms are added,
+they should add corresponding tests in `sel4test`.
+
+Contributions to `master` should additionally either be invisible to the
+proof in https://github.com/seL4/l4v, such as comments, documentation, style,
+unverified platform, etc, or they should come with proof updates to `l4v`.
+
+
+## Contact
+
+If you have larger changes or additions, it might be a good idea to get in
+contact with us as <devel@sel4.systems>, so we can help you get started.