overlay/opkg.mk: add explanatory comment

overlay/opkg.mk

+# LEDE doesn't have a nice way to set the list of feeds in
+# /etc/opkg/distfeeds.conf, so we use this overlay file (which is included by
+# the opkg package Makefile though LEDE's IncludeOverlay mechanism).
+
+# The following definitions make /etc/opkg/distfeeds.conf match the one included
+# in official LEDE builds (by default, FEEDS_DISABLED contains the original list
+# of feeds (which are unused by Gluon), and FEEDS_ENABLED our own feed list).
+
 FEEDS_ENABLED := $(FEEDS_DISABLED)
 FEEDS_DISABLED :=