Add reference to dutch translation in readme

[sjoerdly]

Now that the dutch first version is released, it should be referenced in the readme of the other translations.

Add to spanish readme under heading 'translations' (should be in spanish).

Then for each other translation a bullet in both spanish and target language.

Example options on how to include:

Traducciones

• Inglés (original) / English original (linking to english release)
• Traducción al holandés / Nederlandse vertaling (linking to dutch release)

Otros idiomas

• Measure Outcomes (english) / Inglés (original) (linking to english release)
• Uitkomsten Meten (nederlands) / Traducción al holandés (linking to dutch release)

To be decided: How to describe, what is most clear? which of the above is better, or are there even better ways?