📝 docs(vuepress): add valid-message-syntax rule spec

Author: kazupon

docs/rules/valid-message-syntax.md

@@ -0,0 +1,34 @@
+# vue-i18n/valid-message-syntax
+
+> disallow invalid message syntax
+
+- :star: The `"extends": "plugin:vue-i18n/recommended"` property in a configuration file enables this rule.
+
+This rule warns invalid message syntax.
+
+This rule is useful localization leaks with incorrect message syntax.
+
+## :book: Rule Details
+
+:-1: Examples of **incorrect** code for this rule:
+
+```json
+// ✗ BAD
+{
+  "list-hello": "Hello! {{0}}",
+  "named-hello": "Hello! {{name}}",
+  "linked-hello": "ref:list-hello"
+}
+```
+
+
+:+1: Examples of **correct** code for this rule:
+
+```json
+// ✓ GOOD
+{
+  "list-hello": "Hello! {0}",
+  "named-hello": "Hello! {name}",
+  "linked-hello": "@:list-hello"
+}
+```