Update gitrepo-add.md

Added in an example yaml config when using a private git repo.  This would eliminate confusion as in https://github.com/rancher/fleet/issues/2495

docs/gitrepo-add.md

@@ -40,6 +40,25 @@ Put your private key into secret, use the namespace the GitRepo is in:
 kubectl create secret generic ssh-key -n fleet-default --from-file=ssh-privatekey=/file/to/private/key  --type=kubernetes.io/ssh-auth
 ```
 
+Now the `clientSecretName` must be specified in the repo definition:
+
+```text
+apiVersion: fleet.cattle.io/v1alpha1
+kind: GitRepo
+metadata:
+  name: sample-ssh
+  # This namespace is special and auto-wired to deploy to the local cluster
+  namespace: fleet-local
+spec:
+  # Everything from this repo will be run in this cluster. You trust me right?
+  repo: "git@github.com:rancher/fleet-examples"
+  # or
+  # repo: "ssh://git@github.com/rancher/fleet-examples"
+  clientSecretName: ssh-key
+  paths:
+  - simple
+```
+
 :::caution
 
 Private key with passphrase is not supported.