Add README to explain how schemas can be generated

Author: apelisse

internal/testdata/README.md

@@ -0,0 +1,14 @@
+Generating schemas
+==================
+
+This folder contains a few schemas, which are generally generated by
+using `k8s.io/kube-openapi/cmd/openapi2smd`.
+
+For example, to generate the `k8s-schema.yaml` file, the following
+command can be used:
+
+```
+$ go run k8s.io/kube-openapi/cmd/openapi2smd@latest < \
+  <(curl --silent https://raw.githubusercontent.com/kubernetes/kubernetes/master/api/openapi-spec/swagger.json) \
+  >k8s-schema.yaml
+```