Add variablized example (hashicorp#149)

* Add variablized example 

Maybe this is too much hand holding, but these actions tend to be relatively repetitive wrt inputs that they need and those inputs should pretty much always be variablized.  This PR provides an example of how to variablize things like the working directory and the TF version so they are managed in a single assignment rather than 2-6 assignments.

* Update README.md

Co-Authored-By: Matthew Sanabria <[email protected]>

Co-authored-by: Matthew Sanabria <[email protected]>

Author: sudomateo

README.md

@@ -18,6 +18,9 @@ The most common workflow is to run `terraform fmt`, `terraform init`, `terraform
 name: 'Terraform GitHub Actions'
 on:
   - pull_request
+env:
+  tf_version: 'latest'
+  tf_working_dir: '.'
 jobs:
   terraform:
     name: 'Terraform'
@@ -28,36 +31,36 @@ jobs:
       - name: 'Terraform Format'
         uses: hashicorp/terraform-github-actions@master
         with:
-          tf_actions_version: 0.12.13
+          tf_actions_version: ${{ env.tf_version }}
           tf_actions_subcommand: 'fmt'
-          tf_actions_working_dir: '.'
+          tf_actions_working_dir: ${{ env.tf_working_dir }}
           tf_actions_comment: true
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - name: 'Terraform Init'
         uses: hashicorp/terraform-github-actions@master
         with:
-          tf_actions_version: 0.12.13
+          tf_actions_version: ${{ env.tf_version }}
           tf_actions_subcommand: 'init'
-          tf_actions_working_dir: '.'
+          tf_actions_working_dir: ${{ env.tf_working_dir }}
           tf_actions_comment: true
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - name: 'Terraform Validate'
         uses: hashicorp/terraform-github-actions@master
         with:
-          tf_actions_version: 0.12.13
+          tf_actions_version: ${{ env.tf_version }}
           tf_actions_subcommand: 'validate'
-          tf_actions_working_dir: '.'
+          tf_actions_working_dir: ${{ env.tf_working_dir }}
           tf_actions_comment: true
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - name: 'Terraform Plan'
         uses: hashicorp/terraform-github-actions@master
         with:
-          tf_actions_version: 0.12.13
+          tf_actions_version: ${{ env.tf_version }}
           tf_actions_subcommand: 'plan'
-          tf_actions_working_dir: '.'
+          tf_actions_working_dir: ${{ env.tf_working_dir }}
           tf_actions_comment: true
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}