add tuna (#3378)

README.md

@@ -43,6 +43,12 @@
     python3 -m pip install --find-links https://oneflow-inc.github.io/nightly oneflow_cu90 --user
     ```
 
+  - If you are in China, you could run this to have pip download packages from domestic mirror of pypi:
+    ```
+    python3 -m pip config set global.index-url https://pypi.tuna.tsinghua.edu.cn/simple
+    ```
+    For more information on this, please refer to [pypi 镜像使用帮助](https://mirror.tuna.tsinghua.edu.cn/help/pypi/)
+
   - Support for latest stable version of CUDA will be prioritized. Please upgrade your Nvidia driver to version 440.33 or above and install `oneflow_cu102` if possible. For more information, please refer to [CUDA compatibility documentation](https://docs.nvidia.com/deploy/cuda-compatibility/index.html).
 
   - CPU-only OneFlow is not available for now.