Add .arcconfig for Arcanist support for Phabricator

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@172043 91177308-0d34-0410-b5e6-96231b3b80d8

.arcconfig

@@ -0,0 +1,4 @@
+{
+  "project_id" : "clang-tools-extra",
+  "conduit_uri" : "http://llvm-reviews.chandlerc.com/"
+}