Hi Brandty975,
See this link how to add a devlog easily: https://itch.io/jam/goedware-game-jam-4/topic/1877228/how-to-add-a-developer-log...
You should have it ready before the end of the voting period. It is not necessary to have it ready along with the game, but that would be the ideal situation.
Tell in your description that the devlog is coming and people should check it before the end of the voting period.
We also always send an e-mail before the end of the voting period to attent the participants that a devlog could have been added or updated.