📖 add api/crd update doc (#913)

* add api/crd update doc

Signed-off-by: Omar Farag <omarfarag74@gmail.com>

* make requested changes

Signed-off-by: Omar Farag <omarfarag74@gmail.com>

* Use command instead of editing file

Co-authored-by: Jian Zhu <jiazhu@redhat.com>
Signed-off-by: Omar Farag <omarfarag74@gmail.com>

---------

Signed-off-by: Omar Farag <omarfarag74@gmail.com>
Co-authored-by: Jian Zhu <jiazhu@redhat.com>

CONTRIBUTING.md

@@ -10,6 +10,7 @@
     - [Prerequisites](#prerequisites)
     - [Setting up dev environment](#setting-up-dev-environment)
     - [Building Images](#building-images)
+    - [Making changes to the API and CRDs](#making-changes-to-the-api-and-crds)
     - [Testing the changes in the kind cluster](#testing-the-changes-in-the-kind-cluster)
     - [Integration tests](#integration-tests)
     - [E2E tests](#e2e-tests)
@@ -143,6 +144,54 @@ After building the images, you can push them to the registry by docker/podman pu
  you can use the command `kind load docker-image quay.io/open-cluster-management/<image-name>:test` to load the
   images into the kind cluster.
 
+### Making changes to the API and CRDs
+
+In order to change Custom Resource Definitions and the API in [ocm](https://github.com/open-cluster-management-io/ocm), one needs to make changes to the [api](https://github.com/open-cluster-management-io/api) repository and then use them in the [ocm](https://github.com/open-cluster-management-io/ocm) repository. This section aims to walk you through these steps with an example.
+
+
+1. Clone a fork of both ocm and api to your machine
+    ```shell
+    git clone git@github.com:<your-username>/api.git
+    git clone git@github.com:<your-username>/ocm.git
+    ```
+
+2. Make your API changes
+   
+    Make your changes in your api repository. For example, let's say you wanted to change the `ClientCertExpirationSeconds` to be `ClientCertExpirationMinutes` for some reason:
+
+
+    ```go
+    type RegistrationConfiguration struct {
+      // clientCertExpirationMinutes represents the minutes of a client certificate to expire. If it is not set or 0, the default
+      // duration minutes will be set by the hub cluster. If the value is larger than the max signing duration minutes set on
+      // the hub cluster, the max signing duration minutes will be set.
+      // +optional
+      ClientCertExpirationMinutes int32 `json:"clientCertExpirationMinutes,omitempty"`
+        ...
+    ```
+    In your api repository, run the following to generate CRDs and DeepCopy functions
+    ```shell
+    user@fedora:~/api$ make update
+    ```
+
+    You'll notice that a few files such as `operator/v1/zz_generated.deepcopy.go` and `operator/v1/0000_00_operator.open-cluster-management.io_klusterlets.crd.yaml` have been changed to reflect the changes in the API and CRD.
+3. Sync API updates with ocm repository
+
+    In order to use the new API, you'll have to sync them to your ocm repository. In your ocm repository, go to your go.mod file and replace `open-cluster-management.io/api` with your own updated version. This can be done by using the `replace` directive. At the end of the go.mod file, add:
+
+    ```go
+    go mod edit -replace open-cluster-management.io/api=/home/<user>/api #path to my local api repo w/ changes
+    ```
+
+    _Note: Only use the replace directive during development/debugging. Make sure to remove it before making a PR to [ocm](https://github.com/open-cluster-management-io/ocm) and once changes to the upstream [api repo](https://github.com/open-cluster-management-io/api) are merged._
+
+    Then, in your ocm repository, run the following:
+    ```shell
+    user@fedora:~/ocm$ go mod tidy
+    user@fedora:~/ocm$ go mod vendor
+    user@fedora:~/ocm$ make update
+    ```
+
 ### Testing the changes in the kind cluster
 
 After building and pushing the images, you can test the changes in the kind cluster.