Add a .arcconfig file so that arc works

Test Plan: Used arc to send this diff.

Reviewers: BAILOPAN, psychonic

Differential Revision: https://phabricator.alliedmods.net/D1

--HG--
extra : amend_source : 6aff9a42b440fe57ac096a93e843916c8a64b26d

Author: asherkin

.arcconfig

@@ -0,0 +1,5 @@
+{
+	"project_id" : "sourcemod",
+	"conduit_uri" : "https://phabricator.alliedmods.net/",
+	"history.immutable" : false
+}