docs: add design contribution guidelines

[fivenp]

Summary

You are offering some wonderful design contribution guidelines on your website – which is super rare these days. But we think you might be missing some visibility across Designers.

We're on a mission to enable OSS and Design collaboration via our portal at https://contribute.design

Since there is currently a huge interest by Designers to contribute to OSS we thought you'd love to participate in this initiative and get Nextcloud listed. :)

Now for designers it would of course be ideal if they'd find similar guidelines in each repository.

Ideally we would extend these docs a little bit more in deep (e.g. links to existing resources they can clone, color guides, tokens, etc etc etc) – but I thought this might be a good first start.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[jancborchardt]

Had been on vacation, sorry for the late reply! @fivenp this looks great, thanks for this standardization initiative!

Looks good to me, what do you think @nimishavijay? And any concerns regarding file structure @PVince81?

@fivenp you are also very welcome to post this on the Open Source Design forum at https://discourse.opensourcedesign.net/ :)

[PVince81]

👍

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22