HACKING: Change install command

Creating a symbolic link can result in undesired data loss... Talking
from personal experience after Extension deleted my code appliying an
"update" :'(

HACKING.md

@@ -6,7 +6,8 @@ to start testing your changes is to create a symbolic link:
 ```bash
 $ glib-compile-schemas schemas
 $ mkdir -p ~/.local/share/gnome-shell/extensions
-$ ln -s `pwd` ~/.local/share/gnome-shell/extensions/[email protected]
+$ rm -fr ~/.local/share/gnome-shell/extensions/[email protected] ; \
+ cp -r `pwd` ~/.local/share/gnome-shell/extensions/[email protected]
 ```
 
 And restart GNOME Shell by pressing `Alt+F2` to open the Run Dialog and enter `restart` or `r`.