Tweaks docs to create Istio CRDs first (#717)

* Tweaks docs to create Istio CRDs first

Closes #540

If people are ok with this change I'll make similar modifications to the other
instruction files too.

Signed-off-by: Doug Davis <[email protected]>

* Update Knative-with-IKS.md

Author: Doug Davis

install/Knative-with-IKS.md

@@ -137,8 +137,15 @@ Knative depends on Istio.
 
 1.  Install Istio:
     ```bash
+    kubectl apply --filename https://github.com/knative/serving/releases/download/v0.2.2/istio-crds.yaml && \
     kubectl apply --filename https://github.com/knative/serving/releases/download/v0.2.2/istio.yaml
     ```
+	Note: the resources (CRDs) defined in the `istio-crds.yaml`file are
+	also included in the `istio.yaml` file, but they are pulled out so that
+	the CRD definitions are created first. If you see an error when creating
+	resources about an unknown type, run the second `kubectl apply` command
+	again.
+
 1.  Label the default namespace with `istio-injection=enabled`:
     ```bash
     kubectl label namespace default istio-injection=enabled