Documentation: Enabling eks cluster feature flag

doc/user/project/clusters/add_remove_clusters.md

@@ -206,9 +206,46 @@ GitLab supports:
 Before creating your first cluster on Amazon EKS with GitLab's integration,
 make sure the following requirements are met:
 
+- Enable the `create_eks_clusters` feature flag for your GitLab instance.
 - An [Amazon Web Services](https://aws.amazon.com/) account is set up and you are able to log in.
 - You have permissions to manage IAM resources.
 
+#### Enable the `create_eks_clusters` feature flag **(CORE ONLY)**
+
+NOTE: **Note:**
+If you are running a self-managed instance, EKS cluster creation will not be available
+unless the feature flag `create_eks_clusters` is enabled. This can be done from the Rails console
+by instance administrators.
+
+Use these commands to start the Rails console:
+
+```sh
+# Omnibus GitLab
+gitlab-rails console
+
+# Installation from source
+cd /home/git/gitlab
+sudo -u git -H bin/rails console RAILS_ENV=production
+```
+
+Then run the following command to enable the feature flag:
+
+```
+Feature.enable(:create_eks_clusters)
+```
+
+You can also enable the feature flag only for specific projects with:
+
+```
+Feature.enable(:create_eks_clusters, Project.find_by_full_path('my_group/my_project'))
+```
+
+Run the following command to disable the feature flag:
+
+```
+Feature.disable(:create_eks_clusters)
+```
+
 ##### Additional requirements for self-managed instances
 
 If you are using a self-managed GitLab instance, GitLab must first