Need to register ElasticApmServiceProvider in Lumen 5.5.*

[YuZhenXie]

Would it be better to add following information in README because Lumen doesn't support package discovery which Laravel does? Without this, it can't work in Lumen.

Lumen

Register Service Provider

...
$app->register(\PhilKra\ElasticApmLaravel\Providers\ElasticApmServiceProvider::class);
...

Tested Lumen version: 5.5.

[philkra]

sure, please open a PR for this and we'll merge