docs: add note about issue GKE cluster names

shipperctl's usage of the context name for the Cluster object fails if that context name is not an acceptable Kubernetes object name, as is the case in GKE clusters and their underscores.

This helps people work around the issue in #61, but a better fix will be for shipperctl to give immediate feedback when it detects that issue.

docs/operations/shipperctl.rst

@@ -60,7 +60,7 @@ Here is a minimal configuration to set up a local *minikube* instance:
   - name: minikube
   applicationClusters:
   - name: minikube
-      region: eu-west
+    region: eu-west
 
 This way, setting up an environment to run Shipper in *Docker For Desktop*, for example, is as easy as creating a list of ``managementClusters`` and a list of ``applicationClusters``, and specifying ``docker-for-desktop`` as the name.
 
@@ -76,4 +76,21 @@ Here is something more interesting: having 2 application clusters, and marking o
   - name: eu-2
     region: eu-west
     scheduler:
-    unschedulable: true
+      unschedulable: true
+
+If you're running on GKE, your cluster context names are likely to have underscores in them, like this: ``gke_ACCOUNT_ZONE_CLUSTERNAME``. ``shipperctl``'s usage of the context name as the name of the Cluster object will break, because Kubernetes objects are not allowed to have underscores in their names. To solve this, specify ``context`` explicitly in ``clusters.yaml``, like so:
+
+.. code-block:: yaml
+
+  managementCluster:
+  - name: eu-m # make sure this is a Kubernetes-friendly name
+    context: gke_ACCOUNT_ZONE_CLUSTERNAME_MANAGEMENT # add this
+  applicationClusters:
+  - name: eu-1
+    region: eu-west
+    context: gke_ACCOUNT_ZONE_CLUSTERNAME_APP_1 # same here
+  - name: eu-2
+    region: eu-west
+    context: gke_ACCOUNT_ZONE_CLUSTERNAME_APP_2 # and here
+    scheduler:
+      unschedulable: true