add cachedir instructions to README.md
add cachedir instructions to README.md

xuguozhi/rcnn,rbgirshick/rcnn,liuweiwuxie/rcnn,hycis/rcnn,tangyuxing/context,Jai-Chaudhary/rcnn,golden1232004/rcnn,bikong2/rcnn,gouxiayibu/rcnn,ddx10000/rcnn,girishkvs/rcnn,Lustralisk/rcnn,kevinlin311tw/rcnn,WangHong-yang/rcnn,tangyuxing/context,ddx10000/rcnn,girishkvs/rcnn,yxliang/rcnn,WangHong-yang/rcnn,rbgirshick/rcnn,Jai-Chaudhary/rcnn,golden1232004/rcnn,kevinlin311tw/rcnn,hycis/rcnn,liuweiwuxie/rcnn,Lustralisk/rcnn,yxliang/rcnn,gouxiayibu/rcnn,xuguozhi/rcnn,bikong2/rcnn