added explanation about stdout

exercises/026_hello2.zig

@@ -30,3 +30,11 @@ pub fn main(init: std.process.Init) !void {
     // We just learned of a single statement which can accomplish this.
     stdout.print("Hello world!\n", .{});
 }
+
+// Now you must be thinking "Why would I need to do this instead of good
+// old `std.debug.print()`?", but here's what you need to understand:
+// `std.debug.print()` prints its output to stderr, while the stdout_writer
+// approach prints to stdout.
+// A common practice is to pipe output of a command to other commands,
+// and in order for the piping to work, those commands expect thier input
+// to come from stdout, not stderr.