cmd/config: Add examples and tutorials for config functions

- Add examples under `functions`
- Add built-in tutorial for functions

functions/examples/validator-resource-requests/README.md

@@ -0,0 +1,38 @@
+# Validation
+
+This is an example of implementing a validation function.
+
+This example is written in `go` and uses the `kyaml` libraries for parsing the
+input and writing the output.  Writing in `go` is not a requirement.
+
+## Function implementation
+
+The function is implemented as an [image](image), and built using `make image`.
+    
+The template is implemented as a go program, which reads a collection of input
+Resource configuration, and looks for invalid configuration.
+
+## Function invocation
+
+The function is invoked by authoring a [local Resource](local-resource)
+with `metadata.configFn` and running:
+
+    kustomize config run-fns local-resources/
+    
+This exists non-zero if there is an error.
+
+## Running the Example
+
+Run the validator with:
+
+    kustomize config run-fns local-resource/
+    
+This will return an error:
+
+    cpu-requests missing for container nginx
+
+Now uncomment the resource reservations and run again:
+
+    kustomize config run-fns local-resource/
+    
+This will return success