Todo: Add migration guide from Cake 3 to 4

[burzum]

There is already an unstable 3.0 branch that features CakePHP4 which was provided by @skie. But it still lacks the documentation for upgrading from v2 to v3 of the plugin or in other words from Cake v3 to v4.

We need to update the documentation and add a migration guide.

[waspinator]

it would also be good to update the documentation for using v2.1.

For example the installation page references use Burzum\FileStorage\Lib\FileStorageUtils; which I don't think exists anymore.

[burzum]

@waspinator can you do a pull request for it?