[velux] tweak read me (#14196)

Signed-off-by: Andrew Fiddian-Green <software@whitebear.ch>

bundles/org.openhab.binding.velux/doc/rules.md

@@ -0,0 +1,15 @@
+```java
+// This rule simulates a push button behaviour.
+rule "PushButton of group gV"
+    when
+        Item gV changed
+    then
+        // waiting a second.
+            Thread::sleep(1000)
+        // Foreach-Switch-is-ON
+        gV.allMembers.filter( s | s.state == ON).forEach[i|
+            // switching OFF
+                i.sendCommand(OFF)
+        ]
+    end
+```