Hi @Avijeet Dash
I had a hard time to understand how should I write a Jolt spec before, and wrote this blog post to memorandum what I've learnt based on my experience. It's not an official documentation, but I hope it'll be helpful in someway.
http://ijokarumawak.github.io/nifi/2016/11/22/nifi-jolt/
Thanks,
Koji