Add design to-do button docs

doc/user/project/issues/design_management.md

+---
+stage: Create
+group: Knowledge
+info: "To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers"
+---
+
 # Design Management
 
 > - [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/660) in [GitLab Premium](https://about.gitlab.com/pricing/) 12.2.
@@ -225,6 +231,40 @@ Note that your resolved comment pins will disappear from the Design to free up s
 However, if you need to revisit or find a resolved discussion, all of your resolved threads will be
 available in the **Resolved Comment** area at the bottom of the right sidebar.
 
+## Add To-Do for Designs
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/198439) in GitLab 13.4.
+> - It's [deployed behind a feature flag](../../feature_flags.md), disabled by default.
+> - It's disabled on GitLab.com.
+> - It's not recommended for production use.
+> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](#enable-or-disable-the-design-to-do-button). **(CORE ONLY)**
+
+CAUTION: **Warning:**
+This feature might not be available to you. Check the **version history** note above for details.
+
+Add a To-Do for a design by clicking **Add a To-Do** on the design sidebar:
+
+![To-Do button](img/design_todo_button_v13_4.png)
+
+### Enable or disable the design To-Do button **(CORE ONLY)**
+
+The design To-Do button is under development and not ready for production use. It is
+deployed behind a feature flag that is **disabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can enable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:design_management_todo_button)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:design_management_todo_button)
+```
+
 ## Referring to designs in Markdown
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/217160) in **GitLab 13.1**.