MedO's right, using an Issue Tracker is a good idea. Even if only one person ever uses it (which I doubt will be the case) it makes it easy to assign tasks to specific people, track progress and completion of new features, and focus your development efforts on the most important tasks at any given time.
Also, writing stuff down in a public place increases your transparency. Increased transparency increases interest in the project. It lowers the barrier to entry for new people interested in helping out. It also decreases your
bus factor, which is always a good thing.
GitHub's issue tracker would work well, or Launchpad, or maybe even an open-source license of JIRA onDemand.