add comments in default.options
add comments in default.options

hemingcui/xtern_with_paxos,hemingcui/xtern_with_paxos,hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern,hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern,hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern