Add information on onready annotation to C# differences page (#4565)

(cherry picked from commit 2a4dcc40d6)

getting_started/scripting/c_sharp/c_sharp_differences.rst

@@ -123,6 +123,28 @@ This attribute should be used on a `delegate`, whose name signature will be used
 
 See also: :ref:`doc_c_sharp_signals`.
 
+`onready` keyword
+-----------------
+
+GDScript has the ability to defer the initialization of a member variable until the ready function
+is called with `onready` (cf. :ref:`doc_gdscript_onready_keyword`).
+For example:
+
+.. code-block:: gdscript
+
+    onready var my_label = get_node("MyLabel")
+
+However C# does not have this ability. To achieve the same effect you need to do this.
+
+.. code-block:: csharp
+
+    private Label _myLabel;
+
+    public override void _Ready()
+    {
+        _myLabel = GetNode<Label>("MyLabel");
+    }
+
 Singletons
 ----------
 

getting_started/scripting/gdscript/gdscript_basics.rst

@@ -1647,8 +1647,10 @@ regardless of whether coroutines were used internally. Note that using
 ``while`` would be redundant here as the ``completed`` signal is only emitted
 when the function didn't yield anymore.
 
-Onready keyword
-~~~~~~~~~~~~~~~
+.. _doc_gdscript_onready_keyword:
+
+`onready` keyword
+~~~~~~~~~~~~~~~~~
 
 When using nodes, it's common to desire to keep references to parts
 of the scene in a variable. As scenes are only warranted to be