docs: add changelog for new/removed keepers

Author: lauraseidler

docs/upgrading_to_v29.0.md

@@ -0,0 +1,49 @@
+# Upgrading to v29.0
+The v29.0 release of *kubernetes-engine* is a backwards incompatible
+release.
+
+### Update variant random ID keepers updated
+
+The v29.0 release updates the keepers for the update variant modules. This will force a recreation of the nodepools.
+
+To avoid this, it is possible to edit the remote state of the `random_id` resource to add the new attributes.
+
+1. Perform a `terraform plan` as normal, identifying the `random_id` resource(s) changing and the new/removed attributes
+```tf
+      ~ keepers     = { # forces replacement
+          + "boot_disk_kms_key"           = ""
+          + "gpu_partition_size"          = ""
+          - "labels"                      = "" -> null
+          + "placement_policy"            = ""
+          - "tags"                        = "" -> null
+            # (19 unchanged elements hidden)
+        }
+        # (2 unchanged attributes hidden)
+    }
+```
+2. Pull the remote state locally: `terraform state pull > default.tfstate`
+3. Back up the original remote state: `cp default.tfstate original.tfstate`
+4. Edit the `random_id` resource(s) to add/remove the attributes from the `terraform plan` step
+```diff
+"attributes": {
+            "b64_std": "pool-02-vb4=",
+            "b64_url": "pool-02-vb4",
+            "byte_length": 2,
+            "dec": "pool-02-48574",
+            "hex": "pool-02-bdbe",
+            "id": "vb4",
+            "keepers": {
+            ...
+              "taints": "",
+-             "labels": "",
+-             "tags": "",
++             "boot_disk_kms_key": "",
++             "gpu_partition_size": "",
++             "placement_policy": "",
+            },
+            "prefix": "pool-02-"
+          }
+```
+1. Bump the serial number at the top
+2. Push the modified state to the remote `terraform state push default.tfstate`
+3. Confirm the `random_id` resource(s) no longer changes (or the corresponding `nodepool`) in a `terraform plan`