terraform: update README (#7934)

Add better usage instructions for the plugin which actually work

plugins/terraform/README.md

@@ -10,6 +10,12 @@ Current as of Terraform v0.11.7
 
 ### Usage
 
+To use it, add `terraform` to the plugins array of your `~/.zshrc` file:
+
+```shell
+plugins=(... terraform)
+```
+
  * Type `terraform` into your prompt and hit `TAB` to see available completion options
 
 ### Expanding ZSH prompt with current Terraform workspace name
@@ -17,7 +23,6 @@ Current as of Terraform v0.11.7
 If you want to get current Terraform workspace name in your ZSH prompt open
 your .zsh-theme file and in a chosen place insert:
 
-```
-$FG[045]\
-$(tf_prompt_info)\
+```shell
+PROMPT=$'%{$fg[white]%}$(tf_prompt_info)%{$reset_color%} '
 ```