Updating guide for new version

[siarheipashkevich]

@gerardojbaez it will be better to create an updating guide for newly versions in future or create an update scripts as in laratrust package.

There are a few breaking changes in new version:
you added canceled_immediately to 'plan_subscriptions' table;
you changed user_id in 'plan_subscriptions' to 'morph';

Could you please describe the other possible breaking changes?

[gerardojbaez]

Polymorphic relations was introduced in version 2.0.0; a major release. This change was needed to properly fix #18. Besides this change and what you have listed, there's no other breaking change in 2.1.0. Please review the "Changed" section under v2.1.0 in Changelog; changes were made internally to some functions.

Regarding having an Update Guide, this is a good idea and will be considered for future major releases.