Updated Usage (markdown)

Usage.md

@@ -231,7 +231,8 @@ The list below is from **master**. Press <kbd>?</kbd> in `nnn` to see the keybin
             a  Select all              A  Invert sel
          p ^P  Copy here            w ^W  Cp/mv sel as
          v ^V  Move here               E  Edit sel list
-         x ^X  Delete                Esc  Send to FIFO
+         x ^X  Delete                  S Listed sel size
+          Esc  Send to FIFO
  
  MISC
         Alt ;  Select plugin           =  Launch app
@@ -248,10 +249,11 @@ Notes:
 2. <kbd>B</kbd> adds a new bookmark in the `bookmarks` dir, <kbd>,</kbd> marks a location in-memory
 3. At the filter prompt, <kbd>^L</kbd> toggles last filter and unfilter. If no last filter, exits filter mode and redraws.
 4. <kbd>Q</kbd> quits with `EXIT_FAILURE` if no selection, else prints the selection to stdout and quits with `EXIT_SUCCESS`.
-5. Help & settings, file details and archive listing are shown in the `$PAGER`.
-6. To change shortcuts modify key bindings in `nnn.h` and compile. Option `-K` detects collisions.
-7. Repeat sort order key <kbd>^T</kbd> to apply sort by time, size and default in the same order.
-8. Colemak users should check out [nnn-colemak](https://gitlab.com/Queer_Bird/nnn-colemak).
+5. <kbd>S</kbd> show the total size of non-filtered selected files listed in a directory. For directories, only the size of the directory is added by default. To add the size of the contents of a directory, switch to du mode.
+6. Help & settings, file details and archive listing are shown in the `$PAGER`.
+7. To change shortcuts modify key bindings in `nnn.h` and compile. Option `-K` detects collisions.
+8. Repeat sort order key <kbd>^T</kbd> to apply sort by time, size and default in the same order.
+9. Colemak users should check out [nnn-colemak](https://gitlab.com/Queer_Bird/nnn-colemak).
 
 #### Graphical map