add git ignore file.
add git ignore file.

JeeLiu/BMReachability,Ianutil/BMReachability,ShengQiangLiu/BMReachability,pjk1129/BMReachability,zhouwude/BMReachability,pjk1129/BMReachability,ShengQiangLiu/BMReachability,zhouwude/BMReachability,Ianutil/BMReachability,ShengQiangLiu/BMReachability,JeeLiu/BMReachability,justinjing/BMReachability,zhouwude/BMReachability,pjk1129/BMReachability,justinjing/BMReachability,mingweizhang/BMReachability,Ianutil/BMReachability,JeeLiu/BMReachability,mingweizhang/BMReachability,justinjing/BMReachability,mingweizhang/BMReachability