add mentions of the different placeholders possible for gravatar urls, mentionning the new %{email}

config/gitlab.yml.example

@@ -108,6 +108,7 @@ production: &base
   ## Gravatar
   gravatar:
     enabled: true                 # Use user avatar image from Gravatar.com (default: true)
+    # gravatar urls: possible placeholders: %{hash} %{size} %{email}
     # plain_url: "http://..."     # default: http://www.gravatar.com/avatar/%{hash}?s=%{size}&d=mm
     # ssl_url:   "https://..."    # default: https://secure.gravatar.com/avatar/%{hash}?s=%{size}&d=mm
 
@@ -224,4 +225,4 @@ test:
       new_issue_url: "http://redmine/projects/:issues_tracker_id/issues/new"
 
 staging:
-  <<: *base
+  <<: *base
\ No newline at end of file