Install generated plugin code into the API module.

plugin/README.md

@@ -0,0 +1,27 @@
+This directory and the directories below it
+are not _importable_ Go packages.
+
+Each directory contains a kustomize plugin,
+which is either 
+
+* some random non-Go based executable, 
+  e.g. a bash script that runs java
+  in a JVM, or python in a python VM, etc.
+
+* the source code for a Go executable,
+  i.e. a `main.go` in the unimportable
+  `main` package,
+  ideally declaring its dependencies
+  with it's own `go.mod` file.
+
+* the source code for a Go
+  plugin, which is also an unimportable
+  `main` package ideally with its
+  own `go.mod` file, formulated to be
+  a Go plugin.  If in the `builtin`
+  sub-directory, these plugins are converted
+  to statically linkable code.
+
+To read more about plugins, see
+sigs.k8s.io/kustomize/api/plugins/doc.go
+