Add Howto page (to explain how to use 'Odoo user connect' feature)

Howto.md

+## Add odoo user login button
+
+- In the template, include "conn_admin.html" as following :
+
+```
+{% block content %}
+  {% include "common/conn_admin.html" %}
+```
+
+- In the JS code, use the following pattern (for example) :
+```
+$(document).ready(function() {
+    if (coop_is_connected()) {
+     .....
+    }
+}
+```
\ No newline at end of file