[gtk-web: 1/4] Update vala.md with compile commands for gtk4

[Emmanuele Bassi <ebassi src gnome org>]

commit 8e7f0e8524d7c39981a80f5525cd4693488e1d61
Author: ffwqo <dark cakelord gmail com>
Date:   Thu Jun 24 11:12:15 2021 +0000

    Update vala.md with compile commands for gtk4

 _docs/language-bindings/vala.md | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/_docs/language-bindings/vala.md b/_docs/language-bindings/vala.md
index ffcef61..d61c95a 100644
--- a/_docs/language-bindings/vala.md
+++ b/_docs/language-bindings/vala.md
@@ -23,6 +23,7 @@ You can see many Vala and GTK based projects[GNOME Wiki](https://wiki.gnome.org/
 ## A Hello World app
 Below is an `Hello World` program that can be used as an example to get started with GTK Vala binding.
 ```vala
+//hello_world.vala
 int main (string[] argv) {
     // Create a new application
     var app = new Gtk.Application ("com.example.GtkApplication",
@@ -48,6 +49,11 @@ int main (string[] argv) {
 }
 ```
 
+To compile the program use:
+```
+valac --pkg gtk4 hello_world.vala
+```
+
 ## Contribute
 
 If you are interested in contributing to the Vala and GTK binding project, you can get a head start by 
reading the instructions on how to get started for contributing to Vala 
[here](https://wiki.gnome.org/Projects/Vala/Bindings).