Add a link to the Ruby Regexp documentation

app/views/projects/git_hooks/index.html.haml

@@ -37,7 +37,12 @@
     = f.label :commit_message_regex, "Commit message", class: 'control-label'
     .col-sm-10
       = f.text_field :commit_message_regex, class: "form-control", placeholder: 'Example: Fixes \d+\..*'
-      %p.hint Commit message must match this regular expression to be pushed. If this field is empty it allows any commit message. For example you can require that an issue number is always mentioned in the commit message.
+      %p.hint
+        All commit messages must match this
+        = link_to 'Ruby regular expression', 'http://www.ruby-doc.org/core-2.1.1/Regexp.html'
+        to be pushed.
+        If this field is empty it allows any commit message.
+        For example you can require that an issue number is always mentioned in the commit message.
 
   .form-actions
     = f.submit "Save Git hooks", class: "btn btn-create"