The Digger's Diary was a unique feature of the Wii U version of Shovel Knight that was integrated with the Miiverse. Players would choose an avatar and then can write or draw posts through the tablet. Posts were bound to the rooms they were written in, allowing players to communicate about challenges and secrets in the area.
The service is no longer functional as Nintendo Shutdown Miiverse as of November 8th 2017.
See Also Edit
http://yachtclubgames.com/2014/06/diggers-diary-detailed/ for the design process behind the Digger's Diary.