docs: add migration note for deprecation of ubuntu 16.04
docs: add migration note for deprecation of ubuntu 16.04

dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku