update contribution guidelines

 * the seL4 foundation requires DCO process instead of a CLA
 * minor document re-org and markdown lint fixes

Signed-off-by: Gerwin Klein <[email protected]>

Author: Gerwin Klein

CONTRIBUTING.md

@@ -4,48 +4,48 @@
      SPDX-License-Identifier: CC-BY-SA-4.0
 -->
 
-Contributions to the seL4 kernel repository are welcome!
-
-
-## CLA
-
-Please note that GD and Data61 require a contributor license agreement (CLA)
-to be signed for all external contributions. Please see
-[1] 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.
+# Contributions Welcome!
 
+Contributions to the seL4 kernel repository are welcome!
 
 ## Kernel Development Process
 
-In addition to our guidelines[1], the kernel has additional history requirements:
+In addition to our general [contribution guidelines][1], the kernel has additional git history requirements:
 
 * Please try to make sure every commit is in a working state to facilitate bisecting.
     + unless there is a concrete reason, if so please state that reason in the commit message.
 * Try to keep commits small for ease of reviewing.
 
+[1]: https://docs.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,
+<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,
+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 <[email protected]>, so we can help you get started.
+If you have larger changes or additions, it is a good idea to get in contact
+with us as <[email protected]>, so we can help you get started.
+
+## Developer Certificate of Origin (DCO)
+
+This repository uses the same sign-off process as the Linux kernel. For every
+commit, use
+
+    git commit -s
+
+to add a sign-off line to your commit message, which will come out as:
+
+    Signed-off-by: name <email>
+
+By adding this line, you make the declaration that you have the right to make
+this contribution under the open source license the files use that you changed
+or contributed.
 
-[1] https://docs.sel4.systems/Contributing
+The full text of the declaration is at <https://developercertificate.org>.