Use a prefix key to execute the key handler

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.

README.md

@@ -113,6 +113,10 @@ of small previews is displayed, making it easy to choose an image to open.
     N            Go [count] marked images forward
     P            Go [count] marked images backward
 
+    Ctrl-x KEY	 Execute $XDG_CONFIG_HOME/sxiv/exec/key-handler with
+                 KEY and the path of the current image as arguments.
+		 If KEY is Escape, it cancels the effect of Ctrl-x.
+
 *Thumbnail mode:*
 
     h,j,k,l      Move selection left/down/up/right [count] times