Improve introductory paragraph

We want to draw attention to the specific syntax of the tags to ensure
the right ones are chosen. Hopefully this should avoid the issue we
experienced recently where we had to change one of the live tags on the
site: alphagov/publishing-api#1910

source/manual/add-translation-whitehall.html.md

@@ -6,11 +6,14 @@ layout: manual_layout
 section: Publishing
 ---
 
-Useful links:
-
-- [List of ISO 639-1 codes](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes)
-- [Rails Translation Manager](https://github.com/alphagov/rails_translation_manager)
-- [Internationalisation guide](https://github.com/alphagov/whitehall/blob/master/docs/internationalisation_guide.md)
+Before starting, it’s worth checking the language tag that is being added is correct: it should be compatible with the [IETF Language Tag syntax][] and use correct [ISO 639][] codes (with optional [ISO 15924][] or [ISO 3166][] subtags where appropriate). This is important to ensure maximum compatibility with accessibility feature such as screen readers. Some other useful reading includes [Rails Translation Manager][] and [Whitehall Internationalisation guide][].
+
+[IETF Language Tag syntax]: https://en.wikipedia.org/wiki/IETF_language_tag#Syntax_of_language_tags
+[ISO 639]: https://en.wikipedia.org/wiki/ISO_639-1
+[ISO 15924]: https://en.wikipedia.org/wiki/ISO_15924
+[ISO 3166]: https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
+[Rails Translation Manager]: https://github.com/alphagov/rails_translation_manager
+[Whitehall Internationalisation guide]: https://github.com/alphagov/whitehall/blob/master/docs/internationalisation_guide.md
 
 ### 1. Update Government Frontend