Create UPGRADE.md to help users migrating to 2.0

[Duroth]

In PHP 8.0 and 8.1, plenty of functions had their return values changed from $type|false to $type (i.e. array|false => array). Because these function no longer have a 'falsy' return values on error conditions, Safe v2.0 will no longer include them.

To help users migrate to v2.0 when it becomes stable, we'd like to include a small doc file ("UPGRADE.md" or similarly named) that includes, at minimum, a list of all removed functions, and preferably a list of newly added ones.

This would not be a replacement for the phpstan rules, rather an addition for those who can't or won't use PhpStan, or would simply like to manually test their migrations.