docs(design): add design contribution badge

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

README.md

@@ -2,6 +2,7 @@
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/nextcloud/server/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/nextcloud/server/?branch=master)
 [![codecov](https://codecov.io/gh/nextcloud/server/branch/master/graph/badge.svg)](https://codecov.io/gh/nextcloud/server)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/209/badge)](https://bestpractices.coreinfrastructure.org/projects/209)
+[![Design](https://contribute.design/api/shield/nextcloud/server)](https://contribute.design/nextcloud/server)
 
 **A safe home for all your data.**