read: add scale factor tips for Linux users

Author: love-linger

README.md

@@ -84,6 +84,7 @@ For **Linux** users:
 
 * `xdg-open` must be installed to support open native file manager.
 * Make sure [git-credential-manager](https://github.com/git-ecosystem/git-credential-manager/releases) is installed on your linux.
+* Maybe you need to set environment variable `AVALONIA_SCREEN_SCALE_FACTORS`. See https://github.com/AvaloniaUI/Avalonia/wiki/Configuring-X11-per-monitor-DPI.
 
 ## External Tools