update README.md

Author: yuantailing

README.md

@@ -130,11 +130,18 @@ CUDA and C++ extensions via
 git clone https://github.com/NVIDIA/apex
 cd apex
 # if pip >= 23.1 (ref: https://pip.pypa.io/en/stable/news/#v23-1) which supports multiple `--config-settings` with the same key... 
-pip install -v --disable-pip-version-check --no-cache-dir --no-build-isolation --config-settings "--build-option=--cpp_ext --cuda_ext --parallel 4" ./
+pip install -v --disable-pip-version-check --no-cache-dir --no-build-isolation --config-settings "--build-option=--cpp_ext" --config-settings "--build-option=--cuda_ext" ./
 # otherwise
 pip install -v --disable-pip-version-check --no-cache-dir --no-build-isolation --global-option="--cpp_ext" --global-option="--cuda_ext" ./
 ```
 
+To reduce the build time of APEX, parallel building can be enhanced via
+```bash
+export NVCC_APPEND_FLAGS="--threads 4"
+pip install -v --disable-pip-version-check --no-cache-dir --no-build-isolation --config-settings "--build-option=--cpp_ext --cuda_ext --parallel 8" ./
+```
+When CPU cores or memory are limited, the `--parallel` option is generally preferred over `--threads`. See [pull#1882](https://github.com/NVIDIA/apex/pull/1882) for more details.
+
 APEX also supports a Python-only build via
 ```bash
 pip install -v --disable-pip-version-check --no-build-isolation --no-cache-dir ./