docs/en: describe John Wells' lexical sets for English vowels.

docs/languages/gmw/en.md

+# English
+
+- [Short Vowels](#short-vowels)
+- [Long Vowels](#long-vowels)
+- [Rhotic Vowels](#rhotic-vowels)
+- [Reduced Vowels](#reduced-vowels)
+- [Diphthongs](#diphthongs)
+- [Split Vowels](#split-vowels)
+- [References](#references)
+
+----------
+
+The English language support uses a vowel system based on John Wells' Lexical
+Sets<sup>\[<a href="#ref1">1</a>\]</sup>. These were created by Wells in 1982
+by comparing the Received Pronunciation British (RP) and General American
+(GenAm) accents in use at that time.
+
+## Short Vowels
+
+| Lexical Set | RP    | GenAm |
+|-------------|-------|-------|
+| KIT         | ɪ     | ɪ     |
+| DRESS       | e     | ɛ     |
+| TRAP        | æ     | æ     |
+| LOT         | ɒ     | ɑ     |
+| STRUT       | ʌ     | ʌ     |
+| FOOT        | ʊ     | ʊ     |
+
+## Long Vowels
+
+| Lexical Set | RP    | GenAm |
+|-------------|-------|-------|
+| FLEECE      | iː    | i     |
+| PALM        | ɑː    | ɑ     |
+| THOUGHT     | ɔː    | ɔ     |
+| GOOSE       | uː    | u     |
+
+## Rhotic Vowels
+
+These are vowels that are followed by an `r` that is not part of the next syllable
+when considering the root form of the word containing that vowel.
+
+| Lexical Set | RP    | GenAm |
+|-------------|-------|-------|
+| NURSE       | ɜː    | ɝ     |
+| START       | ɑː    | ɑɹ    |
+| NORTH       | ɔː    | ɔɹ    |
+| FORCE       | ɔː    | oɹ    |
+| CURE        | ʊə̯     | ʊɹ    |
+| NEAR        | ɪə̯     | ɪɹ    |
+| SQUARE      | eə̯     | ɛɹ    |
+
+## Reduced Vowels
+
+These are unstressed vowels that differ from the vowels in the main lexical sets.
+
+| Lexical Set | RP    | GenAm |
+|-------------|-------|-------|
+| HAPPY       | ɪ     | i     |
+| COMMA       | ə     | ə     |
+| LETTER      | ə     | ɚ     |
+
+## Diphthongs
+
+| Lexical Set | RP    | GenAm |
+|-------------|-------|-------|
+| FACE        | eɪ̯     | eɪ̯     |
+| PRICE       | aɪ̯     | aɪ̯     |
+| CHOICE      | ɔɪ̯     | ɔɪ̯     |
+| GOAT        | əʊ̯     | oʊ̯     |
+| MOUTH       | aʊ̯     | aʊ̯     |
+
+## Split Vowels
+
+These are lexical sets defined by John Wells that are merged with other lexical
+sets in both RP and GenAm, so have split from one of those lexical sets and
+merged with the other.
+
+| Lexical Set | RP    | GenAm |
+|-------------|-------|-------|
+| BATH        | ɑː    | æ     |
+| CLOTH       | ɒ     | ɔ     |
+
+## References
+
+1. <a name="ref1"></a> Wikipedia.
+   [Lexical set](https://en.wikipedia.org/wiki/Lexical_set). 2017.
+   Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA).