Cherry-pick #16515 INT8v2 readme to Release 1.4#16686

luotao1 merged 4 commits intoPaddlePaddle:release/1.4PaddlePaddle/Paddle:release/1.4from

bingyanghuang:release/1.4_int8_mkldnn_readmebingyanghuang/Paddle:release/1.4_int8_mkldnn_readmeCopy head branch name to clipboard