Add dependabot configuration for Terraform

This will configure `dependabot` to scan Terraform configurations if they exist
in a repository. If a repository stores a Terraform configuration in a sub-
directory this configuration will need to be modified or an additional
configuration must be added if there still exists a Terraform configuration in
the root directory.

.github/dependabot.yml

@@ -11,3 +11,8 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+
+  - package-ecosystem: "terraform"
+    directory: "/"
+    schedule:
+      interval: "weekly"