CONTRIBUTING: add standard Rebble contributing guide

Author: Sorixelle

CONTRIBUTING.md

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+# Contribution Guidelines
+
+## Developer Certification of Origin (DCO)
+
+To make a good faith effort to ensure licensing criteria are met, this
+project requires the Developer Certificate of Origin (DCO) process to be
+followed.
+
+The DCO is an attestation attached to every contribution made by every
+developer. In the commit message of the contribution, (described more fully
+later in this document), the developer simply adds a `Signed-off-by`
+statement and thereby agrees to the DCO.
+
+When a developer submits a patch, it is a commitment that the contributor has
+the right to submit the patch per the license. The DCO agreement is shown
+below and at http://developercertificate.org/.
+
+```
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+### DCO Sign-Off
+
+The "sign-off" in the DCO is a "Signed-off-by:" line in each commit's log
+message. The Signed-off-by: line must be in the following format:
+
+```
+Signed-off-by: Your Name <[email protected]>
+```
+
+For your commits, replace:
+
+- `Your Name` with your real name (pseudonyms, hacker handles, and the
+  names of groups are not allowed)
+
+- `[email protected]` with the same email address you are using to
+  author the commit (CI will fail if there is no match)
+
+You can automatically add the Signed-off-by: line to your commit body using
+`git commit -s`. Use other commits in the repository as examples.
+
+Additional requirements:
+
+- If you are altering an existing commit created by someone else, you must add
+  your Signed-off-by: line without removing the existing one.