Display if the plugin is out of date

[carlalexander]

There should be a way to know that the plugin is out of date when in the admin

[szepeviktor]

This is short and good example.
https://github.com/conedevelopment/simplepay-gateway/blob/master/src/Updater.php#L114-L119

[carlalexander]

Thanks @szepeviktor 💖

[szepeviktor]

Glad to contribute.