[openstreetmap/openstreetmap-website] documentation: add more info about translating (Issue #3894)

[maro-21]

Because not all of the messages on osm.org can be translated on Translatewiki, I suggest to add this information to the documentation.

https://github.com/openstreetmap/openstreetmap-website/blob/master/README.md

```
* Translation is managed by [Translatewiki](https://translatewiki.net/wiki/Translating:OpenStreetMap). Most of the messages can be translated there. However, some translations come from external sources:
   * GraphHopper: https://github.com/graphhopper/graphhopper/blob/master/docs/core/translations.md
   * Time and date format: https://github.com/svenfuchs/rails-i18n
```

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