docs(bgnotify): match README with current features (#12057)

plugins/bgnotify/README.md

 # bgnotify zsh plugin
 
-cross-platform background notifications for long running commands! Supports OSX and Ubuntu linux.
+cross-platform background notifications for long running commands! Supports OSX and Linux.
 
 Standalone homepage: [t413/zsh-background-notify](https://github.com/t413/zsh-background-notify)
 
-----------------------------------
+---
 
-## How to use!
+## How to use
 
 Just add bgnotify to your plugins list in your `.zshrc`
 
 - On OS X you'll need [terminal-notifier](https://github.com/alloy/terminal-notifier)
   * `brew install terminal-notifier` (or `gem install terminal-notifier`)
-- On ubuntu you're already all set!
-- On windows you can use [notifu](https://www.paralint.com/projects/notifu/) or the Cygwin Ports libnotify package
+- On Linux, make sure you have `notify-send` or `kdialog` installed. If you're using Ubuntu you should already be all set!
+- On Windows you can use [notifu](https://www.paralint.com/projects/notifu/) or the Cygwin Ports libnotify package
 
 
 ## Screenshots
@@ -36,19 +36,26 @@ Just add bgnotify to your plugins list in your `.zshrc`
 One can configure a few things:
 
 - `bgnotify_threshold` sets the notification threshold time (default 6 seconds)
-- `function bgnotify_formatted` lets you change the notification
+- `function bgnotify_formatted` lets you change the notification. You can for instance customize the message and pass in an icon.
 
 Use these by adding a function definition before the your call to source. Example:
 
-~~~ sh
+```sh
 bgnotify_threshold=4  ## set your own notification threshold
 
 function bgnotify_formatted {
   ## $1=exit_status, $2=command, $3=elapsed_time
-  [ $1 -eq 0 ] && title="Holy Smokes Batman!" || title="Holy Graf Zeppelin!"
-  bgnotify "$title -- after $3 s" "$2";
+
+  # Humanly readable elapsed time
+  local elapsed="$(( $3 % 60 ))s"
+  (( $3 < 60 ))   || elapsed="$((( $3 % 3600) / 60 ))m $elapsed"
+  (( $3 < 3600 )) || elapsed="$((  $3 / 3600 ))h $elapsed"
+
+  [ $1 -eq 0 ] && title="Holy Smokes Batman" || title="Holy Graf Zeppelin"
+  [ $1 -eq 0 ] && icon="$HOME/icons/success.png" || icon="$HOME/icons/fail.png"
+  bgnotify "$title - took ${elapsed}" "$2" "$icon"
 }
 
 plugins=(git bgnotify)  ## add to plugins list
 source $ZSH/oh-my-zsh.sh  ## existing source call
-~~~
+```