Add contributing file (#9)

CONTRIBUTING.md

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+# Contributing
+
+Want to contribute? Great! You can do so through the standard GitHub pull
+request model. For large contributions we do encourage you to file a ticket in
+the GitHub issues tracking system prior to any code development to coordinate
+with the system_modes development team early in the process. Coordinating up
+front helps to avoid frustration later on.
+
+Your contribution must be licensed under the Apache-2.0 license, the license
+used by this project.
+
+## Add / retain copyright notices
+
+Include a copyright notice and license in each new file to be contributed,
+consistent with the style used by this project. If your contribution contains
+code under the copyright of a third party, document its origin, license, and
+copyright holders.
+
+## Sign your work
+
+This project tracks patch provenance and licensing using a modified Developer
+Certificate of Origin (DCO; from [OSDL][DCO]) and Signed-off-by tags initially
+developed by the Linux kernel project.
+
+```
+system_modes Developer's Certificate of Origin.  Version 1.0
+
+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 "Apache License, Version 2.0"
+    ("Apache-2.0"); or
+
+(b) The contribution is based upon previous work that is covered by
+    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 Apache-2.0 license;
+    or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a) or (b) 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
+    metadata and personal information I submit with it, including my
+    sign-off) is maintained indefinitely and may be redistributed
+    consistent with this project and the requirements of the Apache-2.0
+    license or any open source license(s) involved, where they are
+    relevant.
+
+(e) I am granting the contribution to this project under the terms of
+    Apache-2.0.
+
+    http://www.apache.org/licenses/LICENSE-2.0
+```
+
+With the sign-off in a commit message you certify that you authored the patch
+or otherwise have the right to submit it under an open source license. The
+procedure is simple: To certify above system_modes Developer's Certificate of
+Origin 1.0 for your contribution just append a line
+
+    Signed-off-by: Random J Developer <random@developer.example.org>
+
+to every commit message using your real name or your pseudonym and a valid
+email address.
+
+If you have set your `user.name` and `user.email` git configs you can
+automatically sign the commit by running the git-commit command with the `-s`
+option.  There may be multiple sign-offs if more than one developer was
+involved in authoring the contribution.
+
+For a more detailed description of this procedure, please see
+[SubmittingPatches][] which was extracted from the Linux kernel project, and
+which is stored in an external repository.
+
+### Individual vs. Corporate Contributors
+
+Often employers or academic institution have ownership over code that is
+written in certain circumstances, so please do due diligence to ensure that
+you have the right to submit the code.
+
+If you are a developer who is authorized to contribute to system_modes on
+behalf of your employer, then please use your corporate email address in the
+Signed-off-by tag. Otherwise please use a personal email address.
+
+## Maintain Copyright holder / Contributor list
+
+Each contributor is responsible for identifying themselves in the
+[NOTICE](NOTICE) file, the project's list of copyright holders and authors.
+Please add the respective information corresponding to the Signed-off-by tag
+as part of your first pull request.
+
+If you are a developer who is authorized to contribute to system_modes on
+behalf of your employer, then add your company / organization to the list of
+copyright holders in the [NOTICE](NOTICE) file. As author of a corporate
+contribution you can also add your name and corporate email address as in the
+Signed-off-by tag.
+
+If your contribution is covered by this project's DCO's clause "(c) The
+contribution was provided directly to me by some other person who certified
+(a) or (b) and I have not modified it", please add the appropriate copyright
+holder(s) to the [NOTICE](NOTICE) file as part of your contribution.
+
+
+[DCO]: http://web.archive.org/web/20070306195036/http://osdlab.org/newsroom/press_releases/2004/2004_05_24_dco.html
+
+[SubmittingPatches]: https://github.com/wking/signed-off-by/blob/7d71be37194df05c349157a2161c7534feaf86a4/Documentation/SubmittingPatches