This adds the deprecation notices for ko:// and nonroot (#161)

* Add a deprecation notice when `defaultBaseImage` is not specified.

Related: https://github.com/google/ko/issues/160

* Add a deprecation notice when `ko://` is not specified.

Related: https://github.com/google/ko/issues/158

* Add a sample import path.

cmd/ko/test/test.yaml

@@ -15,10 +15,8 @@ apiVersion: v1
 kind: Pod
 metadata:
   name: kodata
-  annotations:
-    sidecar.istio.io/inject: "false"
 spec:
   containers:
   - name: obiwan
-    image: github.com/google/ko/cmd/ko/test
+    image: ko://github.com/google/ko/cmd/ko/test
   restartPolicy: Never