Xiaomi has officially released the Kernel source of Redmi 4 Prime.
Individual Android Developer will have a chance to further development/improvement of the ROM.
Moreover, third party custom ROM builders such as Lineage OS, Remix OS,and others will be able to release their official version for Redmi 4 Prime.
With this official release, it is expected that Xiaomi Redmi 4 Prime users will get Android Nougat based custom ROM in near future.
Note that this is not the ROM which you can flash into your device.
Kernel source is a raw material which requires developers to compile and made it into a custom ROM.