Merge branch 'backup-restore-tar-version' into 'master'

Specify tar is required for backup and restore tasks

Closes omnibus-gitlab#2334

See merge request gitlab-org/gitlab-ce!21633

doc/raketasks/backup_restore.md

@@ -16,17 +16,27 @@ and is flexible enough to fit your needs.
 
 ### Requirements
 
-If you're using GitLab with the Omnibus package, you're all set. If you
-installed GitLab from source, make sure the following packages are installed:
-
 * rsync
 
+If you're using GitLab with the Omnibus package, you're all set. If you
+installed GitLab from source, make sure you have rsync installed.
+
 If you're using Ubuntu, you could run:
 
 ```
 sudo apt-get install -y rsync
 ```
 
+* tar
+
+Backup and restore tasks use `tar` under the hood to create and extract
+archives. Ensure you have version 1.30 or above of `tar` available in your
+system. To check the version, run:
+
+```
+tar --version
+```
+
 ### Backup timestamp
 
 >**Note:**