Add documentation for artifact object store

Former-commit-id: 65dfe7f9

changelogs/unreleased/zj-artifacts-object-store.yml

+---
+title: Allow artifacts to be store on object stores
+merge_request: 9860
+author:

config/gitlab.yml.example

@@ -136,6 +136,7 @@ production: &base
   ## Build Artifacts
   artifacts:
     enabled: true
+    object_store: false
     # The location where build artifacts are stored (default: shared/artifacts).
     # path: shared/artifacts

doc/administration/job_artifacts.md

@@ -82,6 +82,15 @@ _The artifacts are stored by default in
 1. Save the file and [restart GitLab][] for the changes to take effect.
+---
+
+**Using Object Store**
+
+The previously mentioned methods use the local disk to store artifacts. However,
+there is the option to use object stores like AWS' S3. To do this, set the
+`object_store` flag to true in your `gitlab.rb`. This relies on valid AWS
+credentials to be configured already.
+
 ## Set the maximum file size of the artifacts
 Provided the artifacts are enabled, you can change the maximum file size of the