document URL shortener option

Configuration.md

@@ -93,6 +93,10 @@ false               | set, i.e. to "en" | set     | will always be in the `langu
 true                | not set           | set     | uses browser language or the one last selected and stores it in the cookie
 true                | set, i.e. to "fr" | set     | uses browser language or the one last selected and stores it in the cookie and uses "fr" as the fallback for unknown browser languages
 
+### urlshortener
+
+Optionally Zerobin can offer a link to a URL shortener service after a new paste is created. It is strongly suggested to only use this with self-hosted shortener services as this will leak the pastes encryption key to the service entered here.
+
 ### model
 
 The sections `[model]` and `[model_options]` control how your pastes are stored. Currently ZeroBin stores its pastes as flat files by default (`[model]` section, `class = zerobin_data`) and the option `dir` in the section `[model_options]` tells it in what folder to store them. Note that this directory needs to be writable for the process running PHP (i.e. your webserver, FPM or CGI process).