Add .arcconfig

Summary: This repo was missing this file, so one had to manually specify the url in order to make phabricator diffs.

Reviewers: mart, #kirigami, davidedmundson, ngraham

Reviewed By: #kirigami, davidedmundson, ngraham

Subscribers: plasma-devel

Tags: #plasma, #kirigami

Differential Revision: https://phabricator.kde.org/D27006