README: document the optional phonological layer (secryst crystals + interscript-ml index)

Author: ronaldtse

README.md

@@ -2,3 +2,17 @@ image:https://github.com/interscript/interscript-ruby/actions/workflows/rake.yml
 # Interscript
 
 Documentation is available directory higher in file README.adoc.
+
+## Phonological layer (optional)
+
+Some transliteration systems need their input vocalized before the map
+can apply (undiacritized Arabic, nikud-less Hebrew, unsegmented Thai).
+The stdlib ships adapters for this: the `secryst` function dispatches
+to a [secryst crystal](https://www.secryst.org) —
+`gem 'secryst'` (Ruby), `pip install secryst`, or `npm i secryst` —
+which resolves a model id through the
+[interscript-ml `models.yaml` index](https://github.com/interscript/interscript-ml)
+and runs sha256-verified ONNX inference locally. The gem has no
+interscript dependency: a crystal can be used standalone for any G2P
+need, and an engine without a crystal simply cannot execute maps that
+declare a vocalization step.