celery-once and Redlock

[NicolasLM]

I hope this issue doesn't come as too pedantic but the README mentions that celery-once uses the Redlock algorithm.

After digging a bit it looks like redis.lock.Lock uses a plain lock on a single instance, whereas Redlock is about using multiple Redis instances in parallel for fault-tolerance.

The confusion probably comes from the fact that both algorithms are presented in the same page in the Redis documentation.

[cameronmaske]

@NicolasLM Totally not pedantic, and good catch. I'll update the docs to reflect this more accurately!

[cameronmaske]

Closed by #87