Add documentation for the Redis cache driver

[euantorano]

Since merging mybb/mybb#3958, we now support Redis as a cache driver. We need to add some documentation for it.

[Ben-MyBB]

I'm wondering if the best way would be to add a list of supported cache drivers and link to the relevant 3rd party documentation? E.G https://redis.io/documentation

[euantorano]

Yes, but also some documentation about how to set it up in config.php (which is easy really).

[Ben-MyBB]

I'll create a list with the relevant documentation and submit a PR over the next few days hoping to have it up in time for .23 release.