chore(ops/terraform): add license information

This is the result of a `"reuse annotate --copyright "The TVL Authors"
--license MIT"` in that directory, making it conformant with the REUSE
Specification:

https://reuse.software/spec

Change-Id: I13e069b4621e8d5ccb7a09c12f772d70dea40a11
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10170
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

ops/terraform/deploy-nixos/README.md

@@ -1,3 +1,9 @@
+<!--
+SPDX-FileCopyrightText: 2023 The TVL Authors
+
+SPDX-License-Identifier: MIT
+-->
+
 deploy-nixos
 ============