Date place holder should be added to the documentation

Ok, I added a 'd' modifier for dates. First parameter is the date, second one is the format. Both are optionnal, default values are 'now' and the format 'd.m.Y' which is used by all DateAttributes in FD. Examples:

• @%d|%@ => 15.03.2017
• @%d[tomorrow]|%@ => 16.03.2017
• @%d[today+6days]|%@ => 21.03.2017
• @%d[now,l jS \of F Y h:i:s A]|%@ => Wednesday 15th of March 2017 02:12:18 PM

(from redmine: issue id 5434, created on 2017-03-16, closed on 2017-04-12)

• Relations:
  • copied_to #5431 (closed)
• Custom Fields:
  • Support contract: None