cmd/config: Add examples and tutorials for config functions

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

functions/examples/template-heredoc-cockroachdb/README.md

@@ -0,0 +1,50 @@
+# Template HereDoc
+
+This is an example of implementing a template function using a heredoc.
+
+This example uses the simplest approach for building abstractions.
+
+## Function implementation
+
+The function is implemented as an [image](image), and built using `make image`.
+    
+The template is implemented as a heredoc, which substitutes environment variables
+into a static string.
+
+This simple implementation uses `kustomize config run-fns wrap --` to perform the
+heavy lifting of implementing the function interface.
+
+- parse functionConfig from stdin into environment variables
+- merge script output with items from stdin
+
+## 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 generates the `local-resources/config` directory containing the template output.
+
+- the template output may be modified by adding fields -- such as initContainers,
+  sidecarConatiners, cpu resource limits, etc -- and these fields will be retained 
+  when re-running `run-fns`
+- the function input `example-use.yaml` may be changed and rerunning `run-fns` will update
+  only the parts changed in the template output.
+
+## Running the Example
+
+Run the config with:
+
+     kustomize config run-fns local-resource/
+     
+This will create the directory
+
+    local-resource/config
+    
+Add an annotation to the StatefulSet Resource and change the replica count of the
+`kind: CockroachDB` Resource in `example-use.yaml`.  Rerun the template:
+
+    kustomize config run-fns local-resource/
+    
+The replica count should be updated, but your annotation should remain.