[indic] Add comment

https://github.com/harfbuzz/harfbuzz/issues/2298#issuecomment-615318654
diff --git a/src/hb-ot-shape-complex-indic.cc b/src/hb-ot-shape-complex-indic.cc
index 3a42851..34972f8 100644
--- a/src/hb-ot-shape-complex-indic.cc
+++ b/src/hb-ot-shape-complex-indic.cc
@@ -1333,6 +1333,7 @@
 	goto reph_move;
       }
     }
+    /* See https://github.com/harfbuzz/harfbuzz/issues/2298#issuecomment-615318654 */
 
     /*       6. Otherwise, reorder reph to the end of the syllable.
      */