Leader-election demo doc

[rbroggi]

This feature is not related to a problem, I just wanted to let the maintainers know that I have created a demo of a distributed scheduler that uses this library along with the leaderelection library. This library allows for a leader-election algorithm to be compatible with any persistent store, in the demo MongoDB is used as a persistent store. It might be interesting for users using this library to know about the leaderelection lib.

Soon I will make a blog post about this use-case.