add shortcut for logbooks & note explaining notability metric

templates/personindex.html

@@ -17,6 +17,7 @@
 </tr>
 {% endfor %}
 </table>
+<p>This is based purely on attendance, not on activities, surveying or usefulness of any kind. But as Woody Allen said: "90% of success is just turning up". It should really be called "Notably recent expoers" as the metric is just a geometric "recency" (1/2 for attending last year, 1/3 for the year before, etc., added up. Display cuttoff is 1/3.).
 
 
 <h2>All expoers</h2>