docs: add simple sum method example

Author: Whipstickgostop

streamerbot/3.api/3.csharp/0.guide/3.custom-methods.md

@@ -15,12 +15,20 @@ You can call your custom method from within the `Execute()`{lang=cs} method or f
 public bool Execute()
 {
     CustomMethod(); // Call your custom method [!code highlight]
+
+    int sum = Add(5, 10); // Call another custom method that takes parameters [!code highlight]
+
     return true;
 }
 
 public void CustomMethod()
 {
-    CPH.LogInfo("This is a custom method!");
+    CPH.LogInfo("This is a custom method!"); // You can even use CPH methods here!
+}
+
+public int Add(int a, int b)
+{
+    return a + b; // This method takes two integers and returns their sum
 }
 ```