Explicitly mention the format of actions outputs (hashicorp#144)

Author: alexjurkiewicz

README.md

@@ -80,8 +80,8 @@ Inputs configure Terraform GitHub Actions to perform different actions.
 
 Outputs are used to pass information to subsequent GitHub Actions steps.
 
-* `tf_actions_output` - The Terraform outputs in JSON format.
-* `tf_actions_plan_has_changes` - Whether or not the Terraform plan contained changes.
+* `tf_actions_output` - The Terraform outputs in (stringified) JSON format.
+* `tf_actions_plan_has_changes` - `'true'` if the Terraform plan contained changes, otherwise `'false'`.
 * `tf_actions_plan_output` - The Terraform plan output.
 
 ## Secrets