added multi-select for customer, project and activity (#1557)

UPGRADING.md

@@ -13,9 +13,10 @@ Perform EACH version specific task between your version and the new one, otherwi
 **New database tables and fields were created, don't forget to [run the updater](https://www.kimai.org/documentation/updates.html).**
 
 - The directory `var/data/invoices/` will be used to store archived invoice files (check file permissions)
-- The default invoice number format changed, if you want back the old one, use `{date}` as format - see [invoice documentation](https://www.kimai.org/documentation/invoices.html)
+- The default invoice number format changed. If you want to use the old one: configure `{date}` as format - see [invoice documentation](https://www.kimai.org/documentation/invoices.html)
 - HTML invoice templates are now treated like other files and offered as download. If you are using relative URLs for including 
-assets (CSS, images) you need to either inline them (see the default templates) or use absolute URLs.  
+assets (CSS, images) you need to either inline them (see the default templates) or use absolute URLs.
+- Invoice templates that use query values `activity.X` or `project.X` should be checked, as multi-select is now possible for filtering  
 
 Permission changes:
 - `history_invoice` - NEW: grants all features of the new invoice archive (by default for all admins)