Clarify contact page.

doc/contact.html

@@ -80,6 +80,13 @@ xD("fyZKB8xv\"FJytmz8.KAB0u52D")
 </p>
 </noscript>
 
+<p><i>
+Note: I cannot reply to GMail, Google Workplace, Outlook or Office365
+mail addresses, since they prefer to mindlessly filter out mails sent
+from small domains using independent mail servers, such as mine. If you
+don't like that, please complain to Google or Microsoft, not me.
+</i></p>
+
 <h2>Copyright</h2>
 <p>
 All documentation is