checkin README to describe precompiled package build procedure

tools/package_tools/README.md

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+# Precompiled package
+
+This document is going to describe the way to build MMDeploy package.
+
+MMDeploy supports
+
+## Prerequisites
+
+- Download and install Miniconda from the [official website](https://docs.conda.io/en/latest/miniconda.html).
+
+- Create conda environments for python 3.6, 3.7, 3.8 and 3.9
+
+  ```shell
+  for PYTHON_VERSION in 3.6 3.7 3.8 3.9
+  do
+    conda create --name mmdeploy-$PYTHON_VERSION python=$PYTHON_VERSION -y
+  done
+  ```
+
+- Prepare MMDeploy dependencies
+
+  Please follow the [build-on-Linux guide](../../docs/en/01-how-to-build/linux-x86_64.md) or [build-on-Windows guide](../../docs/zh_cn/01-how-to-build/linux-x86_64.md) to install dependencies of MMDeploy,
+  including OpenCV, ppl.cv, ONNX Runtime and TensorRT.
+
+## Run precompiled command
+
+- On Linux platform,
+
+  ```shell
+  conda activate mmdeploy-3.6
+  cd the/root/path/of/mmdeploy
+  python tools/package_tools/mmdeploy_builder.py tools/package_tools/configs/linux_x64.yaml .
+  ```
+
+  You will get the precompiled packages `mmdeploy-{version}-linux-x86_64-cuda11.1-tensorrt8.2.3.0` and `mmdeploy-{version}-linux-x86_64-onnxruntime1.8.1` in the current directory if everything's going well.
+
+- On Windows platform, open Anaconda Powershell Prompt from the start menu and execute:
+
+  ```shell
+  conda activate mmdeploy-3.6
+  cd the/root/path/of/MMDeploy
+  python tools/package_tools/mmdeploy_builder.py tools/package_tools/configs/windows_x64.yaml .
+  ```
+
+  When the build procedure finishes successfully, you will find `mmdeploy-{version}-windows-amd64-cuda11.1-tensorrt8.2.3.0` and `mmdeploy-{version}-windows-amd64-onnxruntime1.8.1` precompiled packages in the current directory.