Sony Xperia S has got un-official Android Lollipop ROM by developer mickybart. nAOSP stands for near to AOSP ROM which is basically an AOSP ROM and this is an unofficial build. Means there is no official support from nASOP but the developer is sole responsible to provide you the updates, so, don’t worry.
Installing custom ROM requires a custom recovery to be installed on your device, which may VOID your warranty and may delete all your data. Before proceeding further you may consider backup your all data. You are the only person doing changes to your phone and I cannot be held responsible for the mistakes done by you. Neither the developer nor we will be held responsible for any disaster that happens to your device.
Required files for Xperia S Lollipop ROM installation: