Documentation Should Mention that redis-rs itself supports r2d2 via the r2d2 feature.

[jasongoodwin]

It's very confusing to navigate the eco system at the moment - this project should likely be marked deprecated as these features exist in redis-rs, but the documentation omits mention and calls out that the project does not support pooling at all which is incorrect.

https://github.com/mitsuhiko/redis-rs/pull/388 <- something like this should be merged into the redis-rs project and this project should reference these features likely so that people don't use this project when they could be using redis-rs 0.21 w/ the r2d2 feature.