Update CHANGELOG and doc for locked runner feature

CHANGELOG

@@ -39,6 +39,7 @@ v 8.9.0 (unreleased)
   - Projects pending deletion will render a 404 page
   - Measure queue duration between gitlab-workhorse and Rails
   - Make authentication service for Container Registry to be compatible with < Docker 1.11
+  - Make it possible to lock a runner from being enabled for other projects
   - Add Application Setting to configure Container Registry token expire delay (default 5min)
   - Cache assigned issue and merge request counts in sidebar nav
   - Use Knapsack only in CI environment

doc/ci/runners/README.md

@@ -96,6 +96,12 @@ To register the runner, run the command below and follow instructions:
 sudo gitlab-runner register
 ```
+###  Lock a specific runner from being enabled from other projects
+
+You can configure a runner to prevent it from being enabled for the other
+projects, so it's exclusive to current projects. This setting is available
+on each runner in *Project Settings* > *Runners*.
+
 ###  Making an existing Shared Runner Specific
 If you are an admin on your GitLab instance,