The only relevant rule is "all submissions must be open source and hosted on GitHub"
This does allow you to upload it after completion, as long as all the files are there.
I will say that using github while doing the project is easy to do and error resistant (so a crash doesn't ruin everything). Github desktop has plenty of explanation and isn't too difficult to use: desktop.github.com
You aren't expected to have a commit history or anything, just upload it at the end of the jam. I'll give instructions and stuff on how to do it when I have the time.
