docs: add contribution instructions, require DCO

Signed-off-by: Tommy van der Vorst <tommy@pixelspark.nl>

Author: pixelspark

.vscode/settings.json

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+{
+  "git.alwaysSignOff": true
+}

CONTRIBUTING.md

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+# Contributing Code
+
+High-quality contributions are welcome. Fork the repository and make your changes in a new branch. Once it's ready
+for review, create a pull request.
+
+## Authorship
+
+### The Developer Certificate of Origin (DCO)
+
+Like the Syncthing project, we require the Developer Certificate of Origin (DCO) sign-off on pull requests (PRs). This
+means that all commit messages must contain a signature line to indicate that the developer accepts the DCO.
+
+The DCO is a lightweight way for contributors to certify that they wrote (or otherwise have the right to submit) the code
+and changes they are contributing to the project. Here is the full [text of the DCO](https://developercertificate.org):
+
+---
+
+By making a contribution to this project, I certify that:
+
+1. 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
+
+2. 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
+
+3. The contribution was provided directly to me by some other person who certified (1), (2) or (3) and I have not
+   modified it.
+
+4. 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.
+
+---
+
+Contributors indicate that they adhere to these requirements by adding a `Signed-off-by` line to their commit messages.  
+For example:
+
+    This is my commit message
+
+    Signed-off-by: Random T Developer <randomt@example.org>
+
+The name and email address in this line must match those of the committing author.
+
+## Licensing
+
+All contributions are made available under the same license as the already existing material being contributed to. For
+most of the project and unless otherwise stated this means MPLv2, but there are exceptions (see [README](./README.md)).
+
+Regardless of the license in effect, you retain the copyright to your contribution.