Adds section describing ci_dag_limit_needs usage

doc/ci/yaml/README.md

@@ -2313,6 +2313,23 @@ This example creates three paths of execution:
 - Related to the above, stages must be explicitly defined for all jobs
   that have the keyword `needs:` or are referred to by one.
 
+##### Changing the `needs:` job limit
+
+The maximum number of jobs that can be defined within `needs:` defaults to 10, but
+can be changed to 50 via a feature flag. To change the limit to 50,
+[start a Rails console session](https://docs.gitlab.com/omnibus/maintenance/#starting-a-rails-console-session)
+and run:
+
+```ruby
+Feature::disable(:ci_dag_limit_needs)
+```
+
+To set it back to 10, run the opposite command:
+
+```ruby
+Feature::enable(:ci_dag_limit_needs)
+```
+
 #### Artifact downloads with `needs`
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/14311) in GitLab v12.6.