Document where to find the how to determine the paging group

Point to the right location in the pile of spec
diff --git a/src/bsc_hack.c b/src/bsc_hack.c
index dba7933..9fa0157 100644
--- a/src/bsc_hack.c
+++ b/src/bsc_hack.c
@@ -827,6 +827,7 @@
 		return;
 	}
 
+	/* FIXME: 05.02  6.5.2 Determination of CCCH_GROUP and PAGING_GROUP... */
 	/* get the station to page */
 	pending_station = (struct pending_registered_station*) pending_stations.next;
 	mi_len = generate_mid_from_tmsi(mi, pending_station->tmsi);