overlays/iosevka: document why custom built iosekva is thick

nix/overlays/iosevka/privateBuildPlan.nix

@@ -26,6 +26,8 @@
   };
 
   weights = {
+    # TODO: This is a workaround for iosevka being too thin at small sizes.
+    # This should be possible to do with fontconfig
     # Build 580, but indicate 400 in output
     Regular.shape = 580;
     Regular.menu = 400;