Missing documentation on how to pass variables to terraform

There is currently no documentation on how to pass variables to terraform.

Additionally, it seems like setting environment variables TF_VAR have no effect:
```yaml
name: Production CI

env:
TF_VAR_image_tag: ${{ github.sha }}

on:
push:
branches: [ master ]

jobs:
provision:
runs-on: ubuntu-latest

steps:
- name: Clone the repository code
uses: actions/checkout@v2

- name: Setup the Terraform CLI
uses: hashicorp/setup-terraform@v1
with:
cli_config_credentials_token: ${{ secrets.TF_CREDENTIALS }}
env:
TF_VAR_image_tag: ${{ github.sha }}

- name: Initialize the Terraform working directory
working-directory: ./terraform
id: init
run: terraform init
env:
TF_VAR_image_tag: ${{ github.sha }}

- name: Apply the Terraform execution plan
working-directory: ./terraform
id: plan
run: terraform apply -auto-approve -no-color
env:
TF_VAR_image_tag: ${{ github.sha }}

```