docs: update image resizing

Author: QingWei-Li

docs/helpers.md

@@ -86,7 +86,11 @@ You will get `<a href="/demo/">link</a>`html. Do not worry, you can still set ti
 ```md
 ![logo](https://docsify.js.org/_media/icon.svg ':size=50x100')
 ![logo](https://docsify.js.org/_media/icon.svg ':size=100')
+
+<!-- Support percentage -->
+![logo](https://docsify.js.org/_media/icon.svg ':size=10%')
 ```
 
 ![logo](https://docsify.js.org/_media/icon.svg ':size=50x100')
 ![logo](https://docsify.js.org/_media/icon.svg ':size=100')
+![logo](https://docsify.js.org/_media/icon.svg ':size=10%')