Google launched the fourth Android Developer Preview update earlier this month so it has already been available for a couple of weeks, but if you are the type who likes to get their hands on the Android Open Source Project changelogs as they contain quite a bit of data about what's new, then you're in luck as the changelog has now been posted. While the main bits about the new software were already highlighted during the launch of the updated software as well as in the following days and over the last couple of weeks, this AOSP changelog should provide more of the necessary source code data that might be useful to developers and tinkerers who like to pick around in this sort of information, but it is worth noting that this is not a full changelog and therefore will not list every single change that the new software includes.
If you're a general user who only goes so far as to flash the developer preview so you can check out the new version of Android without waiting for the final and public release or without having to buy buy a new device, the AOSP changelog probably won't do much for you, which is something to keep in mind. For developers though, the changelog does include source code for things like the Android Runtime and various media libraries as well as other stuff that should come in handy.
As stated above you won't find all of the source code for Android N in this changelog, which means there won't be any code for the rumored upcoming new navigation buttons that were leaked just a few days ago. Additionally, any new features which haven't already been confirmed by Google or announced, likely won't be in the changelog either so there is little chance to find anything that hasn't already been made public. If you haven't already flashed Android N and are curious to view the changelog before you install the new software, the source link will have everything listed. This is also not going to be the last changelog before Google pushes the final release software as there are supposed to be five Android N Developer Preview builds, so there should be one more on the way.