Source and Information on GitHub: https://github.com/aduggleby/TrelloActivity
Our trello board: https://trello.com/board/trello-activity-dashboard/4f7d1df0cbcb1a6878bee423
Simply append a hash tag to the url: http://trelloactivity.apphb.com/#displayActivityBoard. Then add that tag to the description of the board. You can add the text anywhere, e.g. "#displayActivityBoard" and "@#displayActivityBoard" will both work.