.arcconfig

internal-doc: document the config register mechanism

This explains the various usage and feature of the config register introduced
in Mercurial 4.3 and 4.4.

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