📝 docs(gitbook): add migration section

gitbook/en/migration.md

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+# Migration
+
+
+## Global Config
+
+### lang
+
+Use `VueI18n` class constructor `locale` option, or `VueI18n#locale` property:
+
+  ```javascript
+  const i18n = new VueI18n({
+    locale: 'en',
+    // ...
+  })
+  const app = new Vue({ i18n }).$mount('#app')
+
+  // change locale
+  i18n.locale = 'ja'
+  // or
+  app.$i18n.locale = 'ja'
+  ```
+
+### fallbackLang
+
+Use `VueI18n` class constructor `fallbackLocale` option, or `VueI18n#fallbackLocale` property:
+
+  ```javascript
+  const i18n = new VueI18n({
+    locale: 'ja',
+    fallbackLocale: 'en',
+    // ...
+  })
+  const app = new Vue({ i18n }).$mount('#app')
+
+  // change fallback locale
+  i18n.fallbackLocale = 'zh'
+  // or
+  app.$i18n.fallbackLocale = 'zh'
+  ```
+
+### missingHandler
+
+Use `VueI18n` class constructor `missing` option, or `VueI18n#missing` property:
+
+  ```javascript
+  const i18n = new VueI18n({
+    // ...
+    missing: (locale, key, vm) => {
+      // handle translation missing
+    },
+    // ...
+  })
+  const app = new Vue({ i18n }).$mount('#app')
+
+  // change missing handler
+  i18n.missing = (locale, key, vm) => {
+    // handle translation missing
+  }
+  // or
+  app.$i18n.missing = (locale, key, vm) => {
+    // handle translation missing
+  }
+  ```
+
+### i18nFormatter
+
+Use `VueI18n` class constructor `formatter` option, or `VueI18n#formatter` property:
+
+  ```javascript
+  class CustomFormatter {
+    format (message, ...values) {
+      // something render logic
+      return 'something string'
+    }
+  }
+
+  const i18n = new VueI18n({
+    // ...
+    fomtter: new CustomFormatter(),
+    // ...
+  })
+  const app = new Vue({ i18n }).$mount('#app')
+
+  // change custom formatter
+  i18n.formatter = {
+    format: (message, ...values) {
+      // something render logic
+      return 'something string'
+    }
+  }
+  // or
+  app.$i18n.formatter = {
+    format: (message, ...values) {
+      // something render logic
+      return 'something string'
+    }
+  }
+  ```
+
+## Global Methods
+
+### Vue.locale
+
+Use `VueI18n` class constructor `messages` option, or `VueI18n#GetLocaleMessage` / `VueI18n#setLocaleMessage` method:
+
+  ```javascript
+  const i18n = new VueI18n({
+    // ...
+    messages: {
+      en: {
+        hello: 'hello world',
+        // ...
+      },
+      ja: {
+        hello: 'こんにちは、世界',
+        // ...
+      }
+    },
+    // ...
+  })
+  const app = new Vue({ i18n }).$mount('#app')
+
+  // get locale message
+  const en = i18n.getLocaleMessage('en')
+  en.greeting = 'hi!'
+  // set locale message
+  i18n.setLocaleMessage('en', en)
+  // or
+  const ja = app.$i18n.getLocaleMessage('ja')
+  ja.greeting = 'やあ！'
+  app.$i18n.setLocaleMessage('ja', ja)
+  ```
+
+### Vue.t
+
+Use `VueI18n#t` method:
+
+  ```javascript
+  const i18n = new VueI18n({
+    locale: 'en',
+    messages: {
+      en: {
+        greeting: 'hi {name}'
+      }
+    },
+    // ...
+  })
+
+  i18n.t('greeting', { name: 'kazupon' }) // -> hi kazupon
+  ```
+
+### Vue.tc
+
+Use `VueI18n#tc` method:
+
+  ```javascript
+  const i18n = new VueI18n({
+    locale: 'en',
+    messages: {
+      en: {
+        apple: 'no apples | one apple | {count} apples'
+      }
+    },
+    // ...
+  })
+
+  const count = 10
+  i18n.tc('apple', count, { count }) // -> 10 apples
+  ```
+
+### Vue.te
+
+Use `VueI18n#te` method:
+
+  ```javascript
+  const i18n = new VueI18n({
+    locale: 'en',
+    messages: {
+      en: {
+        hello: 'hello world'
+      }
+    },
+    // ...
+  })
+
+  i18n.te('hello') // -> true
+  i18n.te('hallo') // -> false
+  i18n.te('hello') // -> true
+  ```
+
+## Constructor Options
+
+### locales
+
+Use `messages` of `VueI18n` class constructor option, or `messages` of `i18n` option (for Component option):
+
+  ```javascript
+  const i18n = new VueI18n({
+    locale: 'en',
+    messages: {
+      en: {
+        greeting: 'hi {name}'
+      }
+    },
+    // ...
+  })
+
+  // for Component
+  const Component1 = {
+    i18n: {
+      messages: {
+        en: {
+          title: 'Title1'
+        }
+      }
+    }
+  }
+  ```
+
+## Instance Properties
+
+### $lang
+
+Use `VueI18n#locale` property:
+
+  ```javascript
+  const i18n = new VueI18n({
+    locale: 'en',
+    // ...
+  })
+  const app = new Vue({ i18n }).$mount('#app')
+
+  // change locale
+  i18n.locale = 'ja'
+  // or
+  app.$i18n.locale = 'ja'
+  ```