Refactor build resources

Move DUB files into /bld/d and add more instructions to README.

src/init/bind.d

@@ -1,6 +0,0 @@
-import std;
-
-void main()
-{
-    printf("Start.");
-}

src/init/d/bind.d

@@ -0,0 +1,30 @@
+import godot;
+// import godot.api.script; // for GodotScript!
+// import godot.api.register; // for GodotNativeLibrary
+// import godot.string; // for gs!
+
+import godot.node;
+
+// minimal class example with _ready method that will be invoked on creation
+class Greeter : GodotScript!Node {
+    @Property String name;
+
+    // this method is a special godot entry point when object is added to the scene
+    @Method
+    void _ready() {
+        // 'gs' is a string wrapper that converts D string to godot string
+        // usually there is helper functions that takes regular D strings and do this for you
+        print(gs!"Hello ", name);
+    }
+}
+
+// register classes, initialize and terminate D runtime, only one per plugin
+mixin GodotNativeLibrary!(
+    // this is a name prefix of the plugin to be acessible inside godot
+    // it must match the prefix in .gdextension file:
+    //     entry_symbol = "mydplugin_gdextension_entry"
+    "mydplugin",
+
+    // here goes the list of classes you would like to expose in godot
+    Greeter,
+);