add comment

vtest2/source/test3.d

@@ -36,6 +36,7 @@ void test3_worker(uint8 n){
   writeln("started worker = ", n);
   bool go_work = true;
   
+  // https://git.4dev.win/221V/dlang_book/src/master/book/13-параллельные-вычисления
   // https://dlang.org/phobos/std_concurrency.html#.receive
   while(go_work){
     //Thread.sleep(dur!"seconds"(120));