add a banner to advertise the survey

diff --git a/doc/eigendoxy_header.html.in b/doc/eigendoxy_header.html.in
index bb149f8..a6b1c1d 100644
--- a/doc/eigendoxy_header.html.in
+++ b/doc/eigendoxy_header.html.in

@@ -20,6 +20,9 @@
 
 </head>
 <body>
+
+<div style="background:#FFDDDD;font-size:120%;text-align:center;margin:0;padding:5px">Please, help us to better know about our user community by answering the following short survey:  <a href="https://forms.gle/wpyrxWi18ox9Z5ae9">https://forms.gle/wpyrxWi18ox9Z5ae9</a></div>
+
 <div id="top"><!-- do not remove this div, it is closed by doxygen! -->
 
 <!--BEGIN TITLEAREA-->