Add comment from GitHub about adding to store as code comment

src/hydra-queue-runner/build-remote.cc

@@ -425,6 +425,19 @@ static void copyPathFromRemote(
     const ValidPathInfo & info
 )
 {
+    // Why both stores? @thufschmitt says:
+    //
+    // > I think it's an easy (and terribly inefficient 😬) way of
+    // making sure that `localStore.queryRealisations` will succeed
+    // (which we IIRC we need later to get back some metadata about the
+    // path to put it in the db).
+    // >
+    // > To be honest, we shouldn't do that but instead carry the needed
+    // metadata in memory until the point where we need it (but that can
+    // come later once we're confident that this is at least correct)
+    //
+    // TODO make the above change to avoid copying excess data back and
+    // forth.
     for (auto * store : {&destStore, &localStore}) {
       /* Receive the NAR from the remote and add it to the
           destination store. Meanwhile, extract all the info from the