Add manual multi-group instructions to the book

This adds manual multi-group instructions to the book.  We'll eventually
get proper multi-group scaffolding.

Author: DirectXMan12

docs/book/src/SUMMARY.md

@@ -40,12 +40,15 @@
   - [Deployment and Testing](./multiversion-tutorial/deployment.md)
 
 ---
-- [User Guides](./user-guide.md)
+
+- [Migrations](./migrations.md)
 
   - [Kubebuilder v1 vs v2](./migration/v1vsv2.md)
 
       - [Migration Guide](./migration/guide.md)
 
+  - [Single Group to Multi-Group](./migration/multi-group.md)
+
 ---
 
 - [Reference](./reference/reference.md)

docs/book/src/migration/guide.md

@@ -83,7 +83,7 @@ kubebuilder create api --group batch --version v1 --kind CronJob
 ```
 
 If you're using multiple groups, some manual work is required to migrate.
-Please follow [this](/TODO.md) for more details.
+Please follow [this](/migration/multi-group.md) for more details.
 
 ### Migrate the APIs
 

docs/book/src/migration/multi-group.md

@@ -0,0 +1,79 @@
+# Single Group to Multi-Group
+
+<aside class="note warning">
+
+<h1>Work In Progress: Manual Work Incoming</h1>
+
+Multi-group scaffolding support is not present in the initial version of
+the KubeBuilder v2 scaffolding (as of KubeBuilder v2.0.0).
+
+Once you switch to a multi-group layout, additional manual work will be
+needed to create new API groups.
+
+Support will eventually be added for scaffolding -- see
+[kubernetes-sigs/kubebuilder#923][multi-group-issue] for more information.
+
+</aside>
+
+While KubeBuilder v2 will not scaffold out a project structure compatible
+with multiple API groups in the same repository by default, it's possible
+to modify the default project structure to support it.
+
+Let's migrate the [CronJob example][cronjob-tutorial].
+
+Generally, we use the prefix for the API group as the directory name. We
+can check `api/v1/groupversion_info.go` to find that out:
+
+```go
+// +groupName=batch.tutorial.kubebuilder.io
+package v1
+```
+
+Then, we'll rename `api` to `apis` to be more clear, and we'll move our
+existing APIs into a new subdirectory, "batch":
+
+```bash
+mv api apis
+mkdir apis/batch
+mv apis/v* batch/
+```
+
+Next, we'll need to update all the references to the old package name. For
+CronJob, that'll be `main.go` and `controllers/cronjob_controller.go`.  If
+you've added additional files to your project, you'll need to track down
+imports there as well.
+
+Finally, we'll add a new line to `PROJECT` that marks this as
+a multi-group project:
+
+```yaml
+version: "2"
+domain: tutorial.kubebuilder.io
+repo: tutorial.kubebuilder.io/project
+multigroup: true
+```
+
+While this option doesn't do anything right now, it'll indicate to
+KubeBuilder in future versions that this is a multi-group project.
+
+Now that we've migrated this to a multi-group project, normal KubeBuilder
+scaffolding won't work (as of KubeBuilder v2.0.0 -- see the warning at the
+top of this page).  You'll need to copy the setup that KubeBuilder
+normally does to add new groups for the moment:
+
+- Copy over `<kind>_types.go` and `groupversion_info.go` to
+  `apis/<group>/<version>`
+
+- Register the new group-version with the Scheme in `main.go`
+
+- Add a new `<kind>_controller.go` file
+
+- Add the controller to the manager in `main.go`
+
+The [CronJob tutorial][cronjob-tutorial] explains each of these changes in
+more detail (in the context of how they're generated by KubeBuilder for
+single-group projects).
+
+[multi-group-issue]: https://github.com/kubernetes-sigs/kubebuilder/issues/923 "KubeBuilder Issue #923"
+
+[cronjob-tutorial]: /cronjob-tutorial/cronjob-tutorial.md "Tutorial: Building CronJob"