Add Arc configuration file

Summary:
Added .arcconfig file.

• Phabricator instance: DevCentral
• Callsign: OPS
• No linter, nor unit tests.

Test Plan: echo '{}' | arc call-conduit user.whoami | jq .response.uri

Reviewers: Kaliiixx, dereckson

Differential Revision: http://devcentral.nasqueron.org/D19