Add default logging values to gitaly installation

Gitlab shell has been moved inside of gitaly, and will begin consuming
gitaly's config.toml instead of gitlab shell's config.yml. This MR adds
instruction for adding those default logging values to gitaly's
config.toml

doc/administration/gitaly/index.md

@@ -222,6 +222,11 @@ Check the directory layout on your Gitaly server to be sure.
 
    [auth]
    token = 'abc123secret'
+
+   [logging]
+   format = 'json'
+   level = 'info'
+   dir = '/var/log/gitaly'
    ```
 
 1. Append the following to `/home/git/gitaly/config.toml` for each respective server: