chore: merge .github/CONTRIBUTING.md and CONTRIBUTING

CONTRIBUTING.md

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+# Contribution Guidelines
+
+We will always have a need for developers to help us improve Solder. There is no such thing as a perfect project and
+things can always be improved. If you are a developer and are interested in helping then please do not hesitate. Just
+make sure you follow our guidelines.
+
+## General steps
+
+1. Set up your workspace as described in [Getting Started](https://docs.solder.io/reference/getting-started).
+
+2. Check for existing issues in the [TechnicSolder](https://github.com/TechnicPack/TechnicSolder/issues) repository.
+   There is possibly someone else already working on the same thing.
+
+3. If the issue requires a bigger change you may want to submit the issues without the necessary changes first, so we
+   can confirm the issue and know that you're working on fixing it. You should also create a WIP (work in process) pull
+   request prefixed with ``[WIP]`` early so we can already start reviewing them.
+
+4. Fork the project, clone it and make your changes in an extra branch with a proper branch name ``bugfix-``,
+   ``patch-``, ``feature-``.
+
+5. Test your changes using phpunit, commit and push them to your fork.
+
+6. Submit the pull request to the dev branch with a short summary what you've changed and why it should be changed in
+   that way.
+
+7. If you make additional changes, push new commits to your branch. **Do not squash your changes**, that makes it
+   extremely hard to see what you've changed compared to the previous version of your pull request.
+
+## Developer's Certificate of Origin
+
+This repository uses the Developer's Certificate of Origin:
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+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.
+```