Merge branch 'docs-add-assign-runner' into 'master'

Add 'Assigning a Runner to another project'

Closes #35377

See merge request !13952

doc/ci/runners/README.md

@@ -107,9 +107,26 @@ To lock/unlock a Runner:
 1. Check the **Lock to current projects** option
 1. Click **Save changes** for the changes to take effect
 
+## Assigning a Runner to another project
+
+If you are Master on a project where a specific Runner is assigned to, and the
+Runner is not [locked only to that project](#locking-a-specific-runner-from-being-enabled-for-other-projects),
+you can enable the Runner also on any other project where you have Master permissions.
+
+To enable/disable a Runner in your project:
+
+1. Visit your project's **Settings ➔ Pipelines**
+1. Find the Runner you wish to enable/disable
+1. Click **Enable for this project** or **Disable for this project**
+
+> **Note**:
+Consider that if you don't lock your specific Runner to a specific project, any
+user with Master role in you project can assign your runner to another arbitrary
+project without requiring your authorization, so use it with caution.
+
 ## Protected Runners
 
->**Notes:**
+>
 [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13194)
 in GitLab 10.0.