Add .arcconfig

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.com>
Differential Revision: https://phabricator.apertis.org/D5342

.arcconfig

+{
+  "phabricator.uri": "https://phabricator.apertis.org/",
+  "history.immutable": false,
+  "default-reviewers": "#Reviewers",
+  "projects": "apertis_eclipse_plugins",
+  "lint.engine": "ArcanistConfigurationDrivenLintEngine"
+}