Documentation missing WP_REDIS_USERNAME

[sebastianthulin]

Seems to be missing from the documentation. I also tried using ACLs arrays in the WP_REDIS_PASSWORD constant as described. This does not seem to work, as described in github documentation.

Actions needed to improve the documetation on password protected redis instances:

• Update the row with password instructions, with a valid format of username/password combination.
• Update documentation with the (working) WP_REDIS_USERNAME constant.

[tillkruss]

• WP_REDIS_PASSWORD is the way to go, because it's supported with all clients.
• WP_REDIS_USERNAME is a relic that shouldn't exist and only works when using Predis as the client.

Feel free to open a Pull Request with the documentation changes: https://github.com/rhubarbgroup/redis-cache/blob/develop/README.md