Not all stores should be writable by whoever has a reference to them. For example, you might have a store representing the mouse position or the user's geolocation, and it doesn't make sense to be able to set those values from 'outside'. For those cases, we have readable stores.
Click over to the stores.js tab. The first argument to readable is an initial value, which can be null or undefined if you don't have one yet. The second argument is a start function that takes a set callback and returns a stop function. The start function is called when the store gets its first subscriber; stop is called when the last subscriber unsubscribes.