Added command on sync documentation

docs/extensions/commandonsync.md

@@ -0,0 +1,46 @@
+---
+title: Run a Command on Sync
+---
+
+Run a predefined external command upon sync completion.
+
+-   [Extension Source](https://github.com/gpodder/gpodder/blob/master/share/gpodder/extensions/command_on_sync.py)
+
+
+| setting            | value      | description                                 | version           |
+|--------------------|------------|---------------------------------------------|-------------------|
+| command            | string     | command to execute (passed to the shell)    | git master branch |
+
+Introduction
+-----------
+
+One might want to run an external program when all podcast episodes get synchronized to external database (refresh a player's database or enqueue the episode, download episode statistics, run other device-related program like Last.FM scrobbler, etc.). This extension makes it possible to execute any commad.
+
+Usage
+-----
+
+1. In the Extensions tab under Preferences, check "Run a Command on Sync" (in the "Post Sync" section).
+2. In the Advanced Configuration (**Edit Configuration** button), define the `extensions.command_on_sync.command`
+    entry to the command you want to run.
+
+I advise you to write a wrapper script if the command is a bit long. You can also directly edit
+`$GPODDER_HOME/Settings.json` to refine your command.
+
+Example Commands
+--------------
+
+### Linux
+
+```
+zenity --info --width=600 --text="All episodes syncrhonized"
+```
+
+displays all variables in a dialog window (requires zenity).
+
+### Scrobble playback log file to Last.FM (requires rb-scrobbler)
+
+```
+rb-scrobbler -f $SCROBBLER_LOG_FILE -n delete-on-success
+```
+
+You can obtain rb-scrobbler on [Github](https://github.com/jeselnik/rb-scrobbler).