📝 docs(vuepress): add no-missing-key rule documentation

Author: kazupon

docs/rules/README.md

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+# Available Rules
+
+- :star: mark: the rule which is enabled by `vue-i18n/recommended` preset.
+- :black_nib: mark: the rule which is fixable by `eslint --fix` command.
+
+## Possible Errors
+
+| Rule ID | Description |    |
+|:--------|:------------|:---|
+| [vue-i18n/<wbr>no-missing-key](./no-missing-key.html) | disallow missing locale message key at localization methods | :star: |
+

docs/rules/no-missing-key.md

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+# vue-i18n/no-missing-key
+
+> disallow missing locale message key at localization methods
+
+- :star: The `"extends": "plugin:vue-i18n/recommended"` property in a configuration file enables this rule.
+
+This rule warns locale message key missing if the key does not exist in locale messages.
+
+## :book: Rule Details
+
+The methods that can be detected with this rule are as follows:
+
+- `$t`
+- `t`
+- `$tc`
+- `tc`
+
+:-1: Examples of **incorrect** code for this rule:
+
+```js
+const localeMessages = {
+  en: {
+    hello: 'Hello! DIO!'
+  }
+  ja: {
+    hello: 'こんにちは、DIO！'
+  }
+}
+
+const i18n = new VueI18n({
+  locale: 'en',
+  localeMessages
+})
+
+/* ✗ BAD */
+i18n.t('hi')
+```
+
+:+1: Examples of **correct** code for this rule:
+
+```js
+const localeMessages = {
+  en: {
+    hello: 'Hello! DIO!'
+  }
+  ja: {
+    hello: 'こんにちは、DIO！'
+  }
+}
+
+const i18n = new VueI18n({
+  locale: 'en',
+  localeMessages
+})
+
+/* ✓ GOOD */
+i18n.t('hello')
+```