Add note for upgrade to 1.6.
Add note for upgrade to 1.6.

kristijanhusak/laravel-form-builder,mstaack/laravel-form-builder,leohmoraes/laravel-form-builder,tantam/laravel-form-builder,Devitek/laravel-form-builder,theshaunwalker/laravel-form-builder,barryvdh/laravel-form-builder