Update button.md (rescript-react-native#28)

Co-authored-by: Max Thirouin <[email protected]>

Author: enieber

docs/button.md

@@ -3,3 +3,12 @@ id: button
 title: Button
 wip: true
 ---
+
+## Example
+
+```res
+open ReactNative                                                                                                          
+                                                                                                 
+<Button title={"Click me"} onPress={_ => Js.log("tes")} />
+```
+