Link reason for custom mergeConfigFrom() and mergeConfig()

[herpaderpaldent]

A third party applications service provider must implement two costum functions:

mergeConfigFrom() and mergeConfig() the reason for this is the inability of laravel to correctly merge multidimensional arrays: https://medium.com/@koenhoeijmakers/properly-merging-configs-in-laravel-packages-a4209701746d

with the two custom functions we can overcome this limitation without losing other functionality. We should mention this in our readme for 3rd party devs.