docs: add architecture

Author: kazupon

ARCHITECTURE.md

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+# High-level architecture
+
+## Intlify Message Syntax
+
+The Message Syntax specification used in Vue I18n is defined [here](./spec/syntax.ebnf).
+
+## Intlify Message Syntax AST
+
+```mermaid
+flowchart TD
+  Resource
+  Plural
+  Message
+  Text
+  Named
+  List
+  Linked
+  LinkedKey
+  LinkedModifier
+  Literal
+```