Add .arcconfig file. Do not use yet.

Don't use this yet - it's an experiment, a disaster, whatever. Caveat
emptor. I'll probably delete everything. Do not taunt Happy Fun Ball -
you have been warned.
Signed-off-by: Austin Seipp <austin@well-typed.com>

.arcconfig

+{
+  "project.name"        : "ghc",
+  "repository.callsign" : "GHC",
+  "phabricator.uri"     : "https://phabricator.haskell.org",
+}