Add default reviewers to .arcconfig

So I don't have to pass --reviewers anymore to "git phab attach".

Differential Revision: https://phabricator.apertis.org/D802


Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

.arcconfig

 {
   "phabricator.uri": "https:\/\/phabricator.apertis.org\/",
   "lint.engine": "ArcanistConfigurationDrivenLintEngine",
-  "project": "apertis-tests"
+  "project": "apertis-tests",
+  "default-reviewers": "#reviewers"
 }