sfdx: add plugin and completion for sfdx (#6540)

plugins/sfdx/README.md

+# sfdx plugin
+
+This plugin provides autocompletion for the [Salesforce DX](https://developer.salesforce.com/tools/sfdxcli) CLI.
+
+To use it, add `sfdx` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... sfdx)
+```
+
+Original repository: https://github.com/wadewegner/salesforce-cli-zsh-completion