docs(design): add basic design contribution guidelines

Signed-off-by: fivenp <mh@fivenp.com>

DESIGN.md

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+# Nextcloud Design contribution guidelines
+
+## 👋 Welcome
+
+At Nextcloud we want to make sure to have everything in place to enable designers to contribute – making our apps universally accessible and easy to use.
+
+## 🚢 How to contribute design
+
+We have a dedicated page with more in-detail guidelines on our website: 
+https://nextcloud.com/design/
+
+**TL;DR**
+
+1. Check out open [issues](https://github.com/nextcloud/server/issues) here on GitHub (we label them with `design`)
+2. Make sure create publicly accessible assets 
+3. Add your contributions to an issue and we promise we will review your contribution carefully and foster discussions
+
+[This issue](https://github.com/nextcloud/desktop/issues/877) has examples of other apps, some simple mockups, and specifications about the design. In the discussions in the comments there are updates to the design as well.
+
+[This pull request](https://github.com/nextcloud/desktop/pull/1565) by a developer has the implementation of that issue, the changes they made, and more design discussions and adjustments.
+
+**We encourage you to:**
+
+- Get in touch with the team by joining our [public Talk channel](https://cloud.nextcloud.com/call/gqff69i8)