If you need to keep a door that shuts automatically open for a while, but don't have a door stopper available, try using a belt to hold the door open for you.
Step 1: The Door Knob
Wrap the belt around the door knob.
Step 2: The Other End
Tie the other end of the belt to something nearby, make sure its heavy/sturdy enough to hold the door, you don't want to break anything.
Step 3: Thats It
This is an extremely basic idea, but hopefully it will help you out someday.