Normally when the version number before the decimal point changes, you'd expect a big update, but Google has for the most part kept things as they were. The most notable change is the contact profile page, which now uses posts, photos and interactions from Google Plus to keep you better informed about the goings-on with the given contact. You can access your contact's profile page by long-tapping the message bubble when they contact you, then tapping 'View profile'.
There have been a few cosmetic tweaks to make Hangouts a bit more refined. The volume rocker buttons now work properly, allowing you to change the ringer volume when using Hangouts, and the 'Snooze notifications' option can now be found at the top of the navigation pane.
The update isn't available in the Play Store yet, but you can already download the APK file for it at APK Mirror.