Add title to read-only indicator.

IPython/html/templates/notebook.html

@@ -62,7 +62,7 b' data-notebook-path="{{notebook_path}}"'
               <span class="kernel_indicator_name">Kernel</span>
               <i id="kernel_indicator_icon"></i>
             </p>
-            <i id="readonly-indicator" class="navbar-text">
+            <i id="readonly-indicator" class="navbar-text" title='This notebook is read-only'>
                 <span class="fa-stack">
                     <i class="fa fa-save fa-stack-1x"></i>
                     <i class="fa fa-ban fa-stack-2x text-danger"></i>