Add suitable demo example

[Edvinas Jurelė]

Current project index.js file has just simple "Hello, mom!" text. But it may be good to have some working example.

Ideas of examples:

• Timer (preset to 2 mins; start, stop, reset functionality)
• Increment/decrement (some number with two buttons to inc or decr an integer)
• maybe borrow and example from https://reactjs.org/