Add docs about reverse proxies

opentracker.conf.sample

@@ -92,6 +92,16 @@
 #      appear anywhere on your tracker.
 #
 # access.stats_path stats
+#
+# IIc)
+#      If opentracker lives behind one or multiple reverse proxies, you can
+#      every http connection appears to come from these proxies. In order to
+#      take the X-Forwarded-For address instead, compile opentracker with the
+#      WANT_IP_FROM_PROXY option and set your proxy addresses here.
+#
+# access.proxy 10.0.1.23
+# access.proxy 10.0.1.24
+#
 
 # III) Live sync uses udp multicast packets to keep a cluster of opentrackers
 #      synchronized. This option tells opentracker which port to listen for