add instructions about error condition to hamming exercise

[maco]

The implementation notes say that it should only work for lists of the same length but don't say what to do otherwise. Yes, it's discoverable by running the tests, but it's nice to give complete instructions.

[NobbZ]

As far as I remember the changed file is generated from the canonical test datas description

https://github.com/exercism/problem-specifications/blob/main/exercises/hamming/description.md

Therefore any change you make to that file will be reverted by the next sync I will eventually do.

I do not remember which file one actually needs to change to get some appended notes to the README after the download.

@exercism/reviewers (sorry for broad ping, but I can't find the "elder" group currently) do you know where this change would make sense to add?

[ErikSchierboom]

@NobbZ The instructions.append.md file can be used for that purpose (see https://exercism.org/docs/building/tracks/practice-exercises#h-file-docs-instructions-append-md).

[NobbZ]

Thanks Erik!

@maco can you please move the changes over to the mentioned file?