[doc] change the compile option in the README.md

[marvin-Yu]

将文档中启用onednn的编译方式
Compile for CPU optimization: oneDNN + Unified Eigen Thread pool

$ bazel build  -c opt --config=opt  --config=mkl_threadpool --define build_with_mkl_dnn_v1_only=true //tensorflow/tools/pip_package:build_pip_package

Compile for CPU optimization and ABI=0

$ bazel build --cxxopt="-D_GLIBCXX_USE_CXX11_ABI=0" --host_cxxopt="-D_GLIBCXX_USE_CXX11_ABI=0" -c opt --config=opt --config=mkl_threadpool --define build_with_mkl_dnn_v1_only=true //tensorflow/tools/pip_package:build_pip_package

修改成如下内容, 删除--define build_with_mkl_dnn_v1_only=true, 添加--copt=-march=skylake-avx512
Compile for CPU optimization: oneDNN + Unified Eigen Thread pool

$ bazel build  -c opt --config=opt  --config=mkl_threadpool --copt=-march=skylake-avx512 //tensorflow/tools/pip_package:build_pip_package

Compile for CPU optimization and ABI=0

$ bazel build --cxxopt="-D_GLIBCXX_USE_CXX11_ABI=0" --host_cxxopt="-D_GLIBCXX_USE_CXX11_ABI=0" -c opt --config=opt --config=mkl_threadpool --copt=-march=skylake-avx512 //tensorflow/tools/pip_package:build_pip_package