add backup link

Author: KumaTea

README.md

@@ -4,7 +4,9 @@ Extended Python wheels (whl) from the PyPI repository
 This project provides pre-built wheels of popular packages for **ARM** / **ARM64** devices,
 to save your time.
 
-**`pip install <package> -f https://ext.maku.ml/wheels.html`**
+**`pip install <package> -f https://ext.maku.ml/whl/stable.html`**
+
+Backup link: `https://cf.ext.maku.ml/whl/stable.html`
 
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