Add note about transparent MitM proxies

Closes: #121

Author: clelland

index.html

@@ -1997,7 +1997,11 @@ <h2>Privacy Considerations</h2>
       processing the <a>request</a>.  For errors during <a>DNS resolution</a>, a
       NEL report only contains information available from DNS itself.  This
       prevents <a>servers</a> from abusing NEL to collect more information about
-      their users than they already have access to.
+      their users than they already have access to. Note that NEL reports will
+      include a web site's public IP address in the <a>server_ip</a> field,
+      which may not always be known to the service which generates the NEL
+      header, for example if it is behind a load balancer or other transparent
+      MitM proxy.
       </p>
 
       <p class="note">