Add contribution guidelines

Signed-off-by: Tobias Kasper <tobias.kasper@tngtech.com>

Author: kaspertng

CONTRIBUTING.md

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+# Contributing
+
+Contributions are very welcome. The following will provide some helpful guidelines.
+
+## How to contribute
+
+If you want to tackle an existing issue please add a comment to make sure the issue is sufficiently discussed
+and that no two contributors collide by working on the same issue. 
+To submit a contribution, please follow the following workflow:
+
+* Fork the project
+* Create a feature branch
+* Add your contribution
+* Create a Pull Request
+
+### Commits
+
+Commit messages should be clear and fully elaborate the context and the reason of a change.
+Each commit message should follow the following conventions:
+
+* it may use markdown to improve readability on GitHub
+* it should start with a title
+  * less than 70 characters
+  * starting lowercase
+* if the commit is not trivial the title should be followed by a body
+  * separated from the title by a blank line
+  * explaining all necessary context and reasons for the change
+* if your commit refers to an issue, please post-fix it with the issue number, e.g. `#123` or `Resolves #123`
+
+A full example:
+
+```
+Fix calculation of menu state
+
+Due to operator precedence, `$(( $menu_state ± 1 % 3 ))`
+was basically equivalent to `$(( $menu_state ± 1 ))`.
+
+Also change 1-based `menu_state` to 0-based `selected_option_index`
+to be able to directly use modulo arithmetics.
+
+Resolves #7
+```
+
+Furthermore, commits must be signed off according to the [DCO](DCO).

DCO

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+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+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 Methods
+
+The DCO requires a sign-off message in the following format appear on each commit in the pull request:
+
+> Signed-off-by: Random J Developer <random@developer.example.org>
+
+using your real name (sorry, no pseudonyms or anonymous contributions.)
+
+The DCO text can either be manually added to your commit body, or you can add either **-s** or **--signoff** to your usual git commit commands. If you forget to add the sign-off you can also amend a previous commit with the sign-off by running **git commit --amend -s**. If you've pushed your changes to Github already you'll need to force push your branch after this with **git push -f**.
+
+
+#### Alternative Sign-Off Methods in rare cases
+
+
+If it is really no option for you to disclose your real name and email address, there might be a chance that you can get your contribution accepted. In this case please contact the maintainers directly and verify the adherence to the DCO of the contribution manually. This might include quite some legal overhead for both parties.