Add guidance for contributors
Switch to redis-semaphore

According to issue #68435604 (PT) we experience some problems
that might be connected with our locking mechanism.

It seems that redis-semaphore is more stable and popular solution
among Sidekiq + Redis locking mechanisms. Thus, I decided to
give it a try.
Merge pull request #164 from bitzesty/semaphore

Switch to redis-semaphore