With this being a brand new just released version of Android, root is not exactly automatic. You can still achieve root but there are a few extra steps in when it comes to rooting the developer preview. You will need to first have a custom recovery installed. If you flashed the image via fastboot you will have to go ahead and reflash your custom recovery. If you flashed the flashable.zip in recovery then you are still good to go in regards to having a custom recovery installed. You will then want to flash SuperSU by Chainfire. Once you have flashed SuperSU you will need to boot into bootloader mode and flash your device's boot.img. Now that you are rooted you will notice that root does not work the same way as it always has. There is no write access due to root app destruction. Some apps like Titanium Backup will not function with this form of root. Other root apps will. If you are running the Dev build and need root head to the source link below.
Via XDA