Update README to remove mutex language

[awiddersheim]

This change removes references to their being a mutex protecting the underlying Redis connection from concurrent access. This was removed in 5.x1.

[awiddersheim]

Seems like based on the plans laid out in https://github.com/redis/redis-rb/issues/1070, folks should be gravitating toward using redis-client instead of this gem in the future. That still fair to say?

[byroot]

I never had any plan to remove the mutex from redis-rb, it would break way too many apps out there. It's still there: https://github.com/redis/redis-rb/blob/935f64f18d7c37386e9b01db71683f8f3b868cd2/lib/redis.rb#L146-L148

[awiddersheim]

Thanks @byroot.