Add kubeval function example.

functions/examples/validator-kubeval/README.md

@@ -0,0 +1,44 @@
+# Validation
+
+This is an example of implementing a validation function against
+[kubeval](https://github.com/instrumenta/kubeval).
+
+## Function implementation
+
+The function is implemented as an [image](image), and built using `make image`.
+
+The function is implemented as a go program, which reads a collection of input
+Resource configuration, passing each to kubeval.
+
+### Function configuration
+
+A number of settings can be modified for `kubeval` in the function `spec`. See
+the `API` struct definition in [main.go](image/main.go) for documentation.
+
+## Function invocation
+
+The function is invoked by authoring a [local Resource](local-resource)
+with `metadata.configFn` and running:
+
+    kustomize config run local-resource/
+
+This exists non-zero if kubeval detects an invalid Resource.
+
+## Running the Example
+
+Run the validator with:
+
+    kustomize config run local-resource/
+
+This will return an error:
+
+    Resource invalid: (Kind: Service, Name: svc)
+    prots: Additional property prots is not allowed
+    Error: exit status 1
+
+Now fix the typo in [example-use.yaml](local-resource/example-use.yaml) and
+run:
+
+    kustomize config run local-resource/
+
+This will return success (no output).