Add docs for gitlab:packages:migrate task

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/administration/raketasks/packages/migrate.md

+# Packages Migrate Rake Task
+
+## Migrate to Object Storage
+
+After [configuring the object storage](../../maven_repository.md#using-object-storage) for GitLab's packages, you may use this task to migrate existing packages from the local storage to the remote storage.
+
+>**Note:**
+All of the processing will be done in a background worker and requires **no downtime**.
+
+### All-in-one rake task
+
+GitLab provides a rake task that migrates all uploaded packages to object storage.
+
+**Omnibus Installation**
+
+```bash
+gitlab-rake "gitlab:packages:migrate"
+```
+
+**Source Installation**
+
+```bash
+sudo RAILS_ENV=production -u git -H bundle exec rake gitlab:packages:migrate
+```

doc/raketasks/README.md

@@ -15,3 +15,4 @@ comments: false
 - [Import](import.md) of git repositories in bulk
 - [Rebuild authorized_keys file](http://docs.gitlab.com/ce/raketasks/maintenance.html#rebuild-authorized_keys-file) task for administrators
 - [Migrate Uploads](../administration/raketasks/uploads/migrate.md)
+- [Migrate Packages](../administration/raketasks/packages/migrate.md)