feat(terragrunt): add tg alias & autocompletion

plugins/terragrunt/README.md

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+## Terragrunt oh-my-zsh plugin
+
+Plugin for Terragrunt, a thin wrapper over Terraform tool from Hashicorp for managing infrastructure safely and efficiently.
+
+Current as of Terragrunt v0.19.16
+
+### Requirements
+
+ * [Terragrunt](https://github.com/gruntwork-io/terragrunt)
+
+### Usage
+
+To use it, add `terragrunt` to the plugins array of your `~/.zshrc` file:
+
+```shell
+plugins=(... terragrunt)
+```
+
+ * Type `terragrunt` into your prompt and hit `TAB` to see available completion options