Add helpful comments to Makefile

Makefile

+# Makefile for installing PowerDNS version of lbnamed
+
+# Files installed:
+
+#   lbbe           -- load-balancing back end
+#   poller         -- polls load-balanced name hosts
+#   poller.config  -- site specific load-balancing names config
+#   LBCD.pm        -- LBCD protocol module
+
+# Note: PowerDNS must also be configured. See ``PowerDNS
+#       Configuration'' in README.md
 
 NAME=power-lbbe