Update Readme to include new {$fieldname:id} qualifier

Reflecting those two commits:

- https://github.com/twiro/link_preview/commit/9b6270141a8a258b6cbc6922201302fb8cd84300
- https://github.com/twiro/link_preview/commit/d4c9172ccff046ea66f3ebd8801a785635563619

README.md

@@ -33,6 +33,7 @@ For more information, see <http://getsymphony.com/learn/tasks/view/install-an-ex
 
 - Use `{$fieldname}` to include the handle of a field
 - Use `{$fieldname:value}` to include the value instead of the handle
+- Use `{$fieldname:id}` to include the ID of an associated entry
 - Use qualifiers for php date_format to format date and datetime fields:
 	- i.e. /article/`{$date:Y}`/`{$date:m}`/`{$date:d}`/`{$title}`/
 - Use qualifiers for php date_format to format the current system date:
@@ -55,4 +56,4 @@ MIT <http://deuxhuithuit.mit-license.org>
 
 *Voila !*
 
-Come say hi! -> <http://www.deuxhuithuit.com/>
+Come say hi! -> <http://www.deuxhuithuit.com/>