Add note on artifacts browser functionality

Based on the following comment:
https://gitlab.com/gitlab-org/gitlab-ce/issues/12634#note_3359006

[ci skip]

doc/ci/build_artifacts/README.md

@@ -11,6 +11,11 @@ Starting from GitLab 8.4 and GitLab Runner 1.0, the artifacts archive format
 changed to `ZIP`, and it is now possible to browse its contents, with the added
 ability of downloading the files separately.
 
+**Note:**
+The artifacts browser will be available only for new artifacts that are sent
+to GitLab using GitLab Runner version 1.0 and up. You will not be available to
+see the browser for old artifacts already uploaded to GitLab.
+
 ## Enabling build artifacts
 
 _If you are searching for ways to use artifacts, jump to