Add mirror example to README

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Co-authored-by: bytemain <13938334+bytemain@users.noreply.github.com>

Author: Copilot

README.md

@@ -16,3 +16,7 @@ if you want install the free-threaded mode of python, you can select the version
 
 You can configure the mirror by `VFOX_PYTHON_MIRROR` environment variable. The default value
 is `https://www.python.org/ftp/python/`.
+
+```bash
+export VFOX_PYTHON_MIRROR=https://mirrors.huaweicloud.com/python/
+```