add a migration document (#507)

this should've been added ages ago back when `nsxiv` was first
released. but even now, there's still plently of people who
might be looking to migrate.

so add a simple MIGRATION.md which covers some of the
differences to be aware of when migrating.

this does not cover patches and `config.h` since any conflicts
in patches would be evident anyways.

ref: https://codeberg.org/nsxiv/nsxiv/issues/369
ref: https://codeberg.org/nsxiv/nsxiv/issues/506
ref: https://codeberg.org/nsxiv/nsxiv/issues/420

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/507
Reviewed-by: eylles <eylles@noreply.codeberg.org>

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