Add Dutch translation

[GewoonLeonYT]

I'm unsure of the correct interpretation of credits.library, therefore I left a comment there and am submitting this as a draft PR.

[vgmoose]

Thank you very much! I've been meaning to add some info about contributing translations to the readme, but happy that you've already gone for it.

For that line, I did intend support in general, although the ambiguity in English isn't necessarily a bad interpretation either.

As an aside, those credits strings are currently still hardcoded into hb-appstore, but I intend to move them (and the localization of them) into the meta-repo in the future. But, it'd need to be translated either way. We also need to add a section for translator credits. 🙏

[GewoonLeonYT]

I've changed the translation and removed the comment. It should be ready now.

[vgmoose]

Thanks again!