Add comment explaining tacky nature of code

Author: yoff

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -819,6 +819,14 @@ class DataFlowType extends TDataFlowType {
 
 /** A node that performs a type cast. */
 class CastNode extends Node {
+  // We include read- and store steps here to force them to be
+  // shown in path explanations.
+  // This hack is necessary, because we have included some of these
+  // steps as default taint steps, making them be suppressed in path
+  // explanations.
+  // We should revert this once, we can remove this steps from the
+  // default taint steps; this should be possible once we have
+  // implemented flow summaries and recursive content.
   CastNode() { readStep(_, _, this) or storeStep(_, _, this) }
 }