redis-cli: add README (#8214)

plugins/redis-cli/README.md

+# Redis-CLI
+
+This plugin adds [redis-cli](https://redis.io/topics/rediscli) completion, based off of Homebrew completion.
+
+To use it, add `redis-cli` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... redis-cli)
+```
+
+## Requirements
+
+In order to make this work, you will need to have redis installed.
+
+More info on the usage and install: https://redis.io/topics/quickstart