Add descriptions to demos

orx-timer/src/demo/kotlin/DemoRepeat03.kt

@@ -0,0 +1,25 @@
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.extra.timer.repeat
+
+/**
+ * Shows how a `repeat` block can update a variable used
+ * for rendering. In this demo, the `opacity` variable is
+ * reduced on every animation frame, and increased to 1.0
+ * every 2 seconds, creating a pulsating animation effect.
+ */
+fun main() = application {
+    program {
+        var opacity = 0.0
+        repeat(2.0) {
+            opacity = 1.0
+        }
+        extend {
+            drawer.clear(ColorRGBa.PINK)
+            drawer.stroke = ColorRGBa.BLACK.opacify(opacity)
+            drawer.fill = ColorRGBa.WHITE.opacify(opacity)
+            drawer.circle(width / 2.0, height / 2.0, 200.0)
+            opacity *= 0.9
+        }
+    }
+}