Add .arcconfig

Summary:
This makes the 'arc' tool aware of the correct Phabricator URL
without the need to explicitly specify it every time.

Test Plan:
Run a command like 'arc list' from within this repository
and verify it "just works."

Reviewers: dantti, ngraham, ltoscano

Reviewed By: ltoscano

Subscribers: ltoscano, kde-utils-devel

Differential Revision: https://phabricator.kde.org/D18659