If you download the SRD (currently on version 3.0) as a single PDF, the revision history starts on page 8. You can use the core book rules just fine, though - nothing's majorly broken. For example, there's a clarification to the Targeting rules that says, "When a spell or power targets a nearby enemy, ally, or creature, you can’t target yourself unless it is explicitly stated that you can also be a target." That's something that could pretty easily be determined at the table, and if the GM decided that a wizard could zap themselves with lightning fork, it wouldn't break the game.
An example of a more significant change is the ranger's Animal Companion rules, which were updated in 13 True Ways as part of the druid class. The core book's version of the animal companion rules isn't in the SRD - the newer rules from 13TW are.
Also, check out the 13th Age FAQ.