Readme: Extend translation section

[tordans]

This PR adds information that is helpful after ideditor/schema-builder#93 was merged.

Changes:

• Clarify that EN US is never translated in Transifex but in the JSON files only
• Add bold text and sub-lists to structure the section ("scanability")
• Clarify that all presets, fields need to be written in EN US as base language
• Add section on Transifex "Developer Notes" which are extended in Extend Transifex developer notes for label and name about locationSet.include countries ideditor/schema-builder#93

[github-actions]

🍱 Preview the tagging presets of this pull request here: https://pr-825--ideditor-presets-preview.netlify.app/id/dist/#locale=en.