Update terraform-versions.md

Updates for runatlantis#789 (comment)

runatlantis.io/docs/terraform-versions.md

@@ -3,6 +3,7 @@
 You can customize which version of Terraform Atlantis defaults to by setting
 the `--default-tf-version` flag (ex. `--default-tf-version=v0.12.0`).
 
+## Via `atlantis.yaml`
 If you wish to use a different version than the default for a specific repo or project, you need
 to create an `atlantis.yaml` file and set the `terraform_version` key:
 ```yaml
@@ -13,8 +14,16 @@ projects:
 ```
 See [atlantis.yaml Use Cases](repo-level-atlantis-yaml.html#terraform-versions) for more details.
 
+## Via terraform config
+Alternatively, one can use the terraform configuration block's `required_version` key to specify an *exact* version:
+```tf
+terraform {
+  required_version = "0.12.0"
+}
+```
+See [Terraform `required_version`](https://www.terraform.io/docs/configuration/terraform.html#specifying-a-required-terraform-version) for reference.
+
 ::: tip NOTE
 Atlantis will automatically download the version specified.
 :::
 
-