Merge branch 'docs-gitaly-shell-secret' into 'master'

Gitaly servers require the GitLab Shell secret

See merge request gitlab-org/gitlab-ce!21847

doc/administration/gitaly/index.md

@@ -101,6 +101,12 @@ documentation on configuring Gitaly
 authentication](https://gitlab.com/gitlab-org/gitaly/blob/master/doc/configuration/README.md#authentication)
 .
+Gitaly must trigger some callbacks to GitLab via GitLab Shell. As a result, 
+the GitLab Shell secret must be the same between the other GitLab servers and
+the Gitaly server. The easiest way to accomplish this is to copy `/etc/gitlab/gitlab-secrets.json`
+from an existing GitLab server to the Gitaly server. Without this shared secret,
+Git operations in GitLab will result in an API error. 
+
 > **NOTE:** In most or all cases the storage paths below end in `/repositories` which is 
 different than `path` in `git_data_dirs` of Omnibus installations. Check the
 directory layout on your Gitaly server to be sure.