Pretty much a non issue. That's surface rust, and while it has gotten under the paint and crazed the frame, it's not structural. You have plenty of options on giving it an oxalic acid bath (use the search function). From there, you can decide what level of determination you have: touch up, strip and paint (rattle can vs. take it to a shop), or powdercoat.

Before you do anything, figure out if you like this bike. Ride it a couple hundred miles. If it's a keeper, then work on it. As long as you store the frame in a dry place, there is no need to worry about the rust spreading.