Add CRD generator documentation to Readme

README.md

@@ -29,3 +29,41 @@ Then do the following with your openshift/origin pull request:
 1. Drop the TMP commit (pointing glide back at openshift/api#master)
 2. Rerun `hack/update-deps.sh` and update your `bump(*)` commit
 3. It can then be tagged and merged by CI
+
+## generating CRD schemas
+
+Since Kubernetes 1.16, every CRD created in `apiextensions.k8s.io/v1` is required to have a [structural OpenAPIV3 schema](https://kubernetes.io/blog/2019/06/20/crd-structural-schema/). The schemas provide server-side validation for fields, as well as providing the descriptions for `oc explain`. Moreover, schemas ensure structural consistency of data in etcd. Without it anything can be stored in a resource which can have security implications. As we host many of our CRDs in this repo along with their corresponding Go types we also require them to have schemas. However, the following instructions apply for CRDs that are not hosted here as well.
+
+These schemas are often very long and complex, and should not be written by hand. For OpenShift, we provide Makefile targets in [library-go's alpha-build-machinery](https://github.com/openshift/library-go/tree/master/alpha-build-machinery) which generate the schema, built on upstream's [controller-gen](https://github.com/kubernetes-sigs/controller-tools) tool.
+
+If you make a change to a CRD type in this repo, simply calling `make update-codegen-crds` should regenerate all CRDs and update the manifests. If yours is not updated, ensure that the path to its API is included in our [calls to the Makefile targets](https://github.com/openshift/api/blob/release-4.5/Makefile#L17-L29).
+
+To add this generator to another repo:
+1. Vendor `github.com/openshift/library-go` (and ensure that the `alpha-build-machinery` subdirectory is also included in your `vendor`)
+
+2. Update your `Makefile` to include the following:
+```
+include $(addprefix ./vendor/github.com/openshift/library-go/alpha-build-machinery/make/, \
+  targets/openshift/crd-schema-gen.mk \
+)
+
+$(call add-crd-gen,<TARGET_NAME>,<API_DIRECTORY>,<CRD_MANIFESTS>,<MANIFEST_OUTPUT>)
+```
+The parameters for the call are:
+
+1. `TARGET_NAME`: The name of your generated Make target. This can be anything, as long as it does not conflict with another make target. Recommended to be your api name.
+2. `API_DIRECTORY`: The location of your API. For example if your Go types are located under `pkg/apis/myoperator/v1/types.go`, this should be `./pkg/apis/myoperator/v1`.
+3. `CRD_MANIFESTS`: The directory your CRDs are located in. For example, if that is `manifests/my_operator.crd.yaml` then it should be `./manifests`
+4. `MANIFEST_OUTPUT`: This should most likely be the same as `CRD_MANIFESTS`, and is only provided for flexibility to output generated code to a different directory.
+
+You can include as many calls to different APIs as necessary, or if you have multiple APIs under the same directory (eg, `v1` and `v2beta1`) you can use 1 call to the parent directory pointing to your API.
+
+After this, calling `make update-codegen-crds` should generate a new structural OpenAPIV3 schema for your CRDs.
+
+**Notes** 
+- This will not generate entire CRDs, only their OpenAPIV3 schemas. If you do not already have a CRD, you will get no output from the generator.
+- Ensure that your API is correctly declared for the generator to pick it up. That means, in your `doc.go`, include the following:
+  1. `// +groupName=<API_GROUP_NAME>`, this should match the `group` in your CRD `spec`
+  2. `// +kubebuilder:validation:Optional`, this tells the operator that fields should be optional unless explicitly marked with `// +kubebuilder:validation:Required`
+
+For more information on the API markers to add to your Go types, see the [Kubebuilder book](https://book.kubebuilder.io/reference/markers.html)