Added a page about the usage of placeholders
Summary: Added placeholder to the index page
Reviewers: kossebau, VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: jnoack, ngraham, abetts
Tags: KDE Human Interface Guidelines
Maniphest Tasks: T8589
Differential Revision: https://phabricator.kde.org/D12879