Add note of deprecation in old Runners CI API

doc/ci/api/runners.md

 # Runners API
 
+_**Note:** In GitLab 8.0, GitLab CI was merged in GitLab itself. This document
+is deprecated in favor of the [new Runners API](../../api/runners.md)._
+
 ## Runners
 
 ### Retrieve all runners
@@ -74,4 +77,4 @@ Returns:
   "updated_at" : "2015-02-26T11:39:39.232Z",
   "description" : "awesome runner"
 }
-```
+```
\ No newline at end of file