Add instruction about adding SetLocaleFromUrlSegment to $middlewarePriority in README

[pgrenaud]

Add instruction about defining $middlewarePriority in App\Http\Kernel and adding \Exolnet\Translation\Http\Middleware\SetLocaleFromUrlSegment::class to the top of the list (or as closest to the top as possible).

Without that, some part of the app experience wont be translated properly. For instance, if the Authenticate middleware redirect an unauthenticated user to the login page, the user will always end up on the login page in the default language (usually English) even if they were attempting to reach a translated URL.

Therefore, the SetLocaleFromUrlSegment middleware must come before the Authenticate middleware. SetLocaleFromUrlSegment should be at the top of the list, unless you have a specific reason to do otherwise.