add docs to experimental/compute-init

Author: bertiethorpe

docs/experimental/compute-init.md

@@ -2,6 +2,21 @@
 
 See the role README.md
 
+# Changes to image / tofu state
+
+When a compute group has ignore_image_changes set as true in a compute group
+partition within tofu/main.tf, and the image is updated:
+
+Subsequent changes to the tf cluster_image variable for that compute group don’t
+actually result in a change via tofu plan/apply. This is done with the
+lifecycle meta-argument "ignore_changes" in the compute resource.
+
+As part of compute-init, the image_id is templated out to hostvars so that
+ansible will have image_id for each compute node.
+
+WIP:    Attempts to change the cluster image from tofu then act as a target
+        for compute-init to read and rebuild to via slurm control.
+
 # CI workflow
 
 The compute node rebuild is tested in CI after the tests for rebuilding the