.arcconfig

phabricator: include the suggested arc config in the repo

Test Plan:
I uploaded this revision with `arc diff`

Differential Revision: https://phab.mercurial-scm.org/D171

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}