📝 docs(vuepress): add keys-order rule spec

Author: kazupon

docs/rules/keys-order.md

@@ -0,0 +1,46 @@
+# vue-i18n/keys-order
+
+> enforce order of localization keys
+
+- :black_nib: The `--fix` option on the [command line](https://eslint.org/docs/user-guide/command-line-interface#fixing-problems) can automatically fix some of the problems reported by this rule.
+
+This rule aims to enforce ordering of localization keys.
+
+This rule is useful to provide browsability of localization keys.
+
+## :book: Rule Details
+
+:-1: Examples of **incorrect** code for this rule:
+
+```json
+// ✗ BAD
+{
+  "message3": "hi!",
+  "message2": "hey!",
+  "message1": "hello!"
+}
+```
+
+
+:+1: Examples of **correct** code for this rule:
+
+```json
+// ✓ GOOD
+{
+  "message1": "hello!",
+  "message2": "hey!",
+  "message3": "hi!"
+}
+```
+
+## Options
+
+```json
+{
+  "vue-i18n/keys-order": ["error", {
+    "order": "desc"
+  }]
+}
+```
+
+- `order`: Case-sensitive sort order of localization keys. Possible values: `asc|desc`. If you don't set any options, it set to `asc` as default.