Add readlink filter doc

As requested at request pull #9213, I'm proposing updathing this doc. Do you have man pages where I can update this too ?

Thanks

docsite/rst/playbooks_variables.rst

@@ -318,6 +318,10 @@ To expand a path containing a tilde (`~`) character (new in version 1.5)::
 
     {{ path | expanduser }}
     
+To get the real path of a link (new in version 1.8)::
+
+   {{ path | readlink }}
+
 To work with Base64 encoded strings::
 
     {{ encoded | b64decode }}