Add note in Readme about translations not being supported

Problem/Motivation

The issue 📌 Add support for multilingual sites Active raises that this does not support multiple languages, only the default language of the site.
The explanation in that ticket should be addressed in the README to make it clear that not only that the module does not support translations, but why.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes