Document source and update process for templates (#31)

This is so it's clear how these files are managed, and how to update
them,
when working with them in the target repositories.

terraform-plans/files/github/CODEOWNERS

@@ -1,3 +1,9 @@
+# This file is centrally managed as a template file in https://github.com/canonical/solutions-engineering-automation
+# To update the file:
+# - Edit it in the canonical/solutions-engineering-automation repository.
+# - Open a PR with the changes.
+# - When the PR merges, the soleng-terraform bot will open a PR to the target repositories with the changes.
+#
 # These owners will be the default owners for everything in the repo. Unless a
 # later match takes precedence, @canonical/soleng-reviewers will be requested for
 # review when someone opens a pull request.

terraform-plans/files/github/charm_promote.yaml

@@ -1,3 +1,8 @@
+# This file is centrally managed as a template file in https://github.com/canonical/solutions-engineering-automation
+# To update the file:
+# - Edit it in the canonical/solutions-engineering-automation repository.
+# - Open a PR with the changes.
+# - When the PR merges, the soleng-terraform bot will open a PR to the target repositories with the changes.
 name: Promote charm to default track, standard risk levels.
 
 on:

terraform-plans/files/github/charm_release.yaml

@@ -1,3 +1,8 @@
+# This file is centrally managed as a template file in https://github.com/canonical/solutions-engineering-automation
+# To update the file:
+# - Edit it in the canonical/solutions-engineering-automation repository.
+# - Open a PR with the changes.
+# - When the PR merges, the soleng-terraform bot will open a PR to the target repositories with the changes.
 name: Release to Edge
 
 on:

terraform-plans/files/github/snap_promote.yaml

@@ -1,3 +1,8 @@
+# This file is centrally managed as a template file in https://github.com/canonical/solutions-engineering-automation
+# To update the file:
+# - Edit it in the canonical/solutions-engineering-automation repository.
+# - Open a PR with the changes.
+# - When the PR merges, the soleng-terraform bot will open a PR to the target repositories with the changes.
 name: Promote snap to default track, standard risk levels.
 
 on:

terraform-plans/files/github/snap_release.yaml

@@ -1,3 +1,8 @@
+# This file is centrally managed as a template file in https://github.com/canonical/solutions-engineering-automation
+# To update the file:
+# - Edit it in the canonical/solutions-engineering-automation repository.
+# - Open a PR with the changes.
+# - When the PR merges, the soleng-terraform bot will open a PR to the target repositories with the changes.
 name: Publish snap
 
 on: