Adding a minimal example and CodeSandbox demo to README

[gotchane]

Hi @jasonkuhrt, thanks to mainteining this project! It is so useful for me. Let me send a PR about improving README.

When I read the issues such as #170 or #175, I had the same impression about the documentation of this project because I was also a newbie of using react-popover.

So I tryed to add a minimal example and CodeSandbox demo to README to make it easier to understand about react-popover.

Any comments are welcome.

[gotchane]

Sorry, it is the first time to use Prettier CI.. Let me ask about how to use it.

I try to build Prettier CI on my forked repository, This PR created. https://github.com/gotchane/react-popover/pull/1

Is it necessary to merge the PR created by Prettier CI to my PR target branch gotchane:add-example-to-readme ?

[MaggiWuerze]

this helped me a lot when getting started