This release introduced namespaces to avoid collisions with other packages, which is a breaking change as classes provided by the Netatmo SDK are no longer available in the global namespace. For instance, NAThermApiClient becomes Netatmo\Clients\NAThermApiClient. For more information, please see http://php.net/manual/en/language.namespaces.php.
It means the use of Netatmo SDK classes in the global namespace is deprecated. It is still possible to use them so far, so that it won't break your existing code. However, we recommend to update your code as soon as possible to take into account those changes.
It also introduced an autoloader respecting the PSR-4 standards, so you only need to include the src/Netatmo/autoload.php file and you're good to go.