docs: Document 4.x to 5.x and 5.x to 6.x migration paths (nuxt-modules#419)

Author: paulgv

CHANGELOG.md

@@ -56,6 +56,8 @@ All notable changes to this project will be documented in this file. See [standa
 
 ### BREAKING CHANGES
 
+[5.x to 6.x migration path](https://nuxt-community.github.io/nuxt-i18n/migrating.html#upgrading-from-5-x-to-6-x)
+
 * Store module's options have been flattened and renamed
 * The mutations responsible for syncing nuxt-i18n's store module with vue-i18n's
 locale and messages are now disabled by default, you'll need to manually re-enable them in the
@@ -464,6 +466,8 @@ cannot be set via the configuration anymore
 
 ### BREAKING CHANGES
 
+[4.x to 5.x migration path](https://nuxt-community.github.io/nuxt-i18n/migrating.html#upgrading-from-4-x-to-5-x)
+
 * [https://github.com/kazupon/vue-i18n/blob/dev/CHANGELOG.md#800-2018-06-23](https://github.com/kazupon/vue-i18n/blob/dev/CHANGELOG.md#800-2018-06-23)
 
 
@@ -509,6 +513,8 @@ cannot be set via the configuration anymore
 
 ### BREAKING CHANGES
 
+[3.x to 4.x migration path](https://nuxt-community.github.io/nuxt-i18n/migrating.html#upgrading-from-3-x-to-4-x)
+
 * Pages using i18n key need to be updated to use nuxtI18n key instead
 
 

docs/migrating.md

@@ -2,6 +2,61 @@
 
 Follow this guide to upgrade from one major version to the other.
 
+
+## Upgrading from 5.x to 6.x
+
+### Global SEO features are now disabled by default
+
+In some cases, having SEO enabled globally caused performance issues and/or conflicted with other plugins. To mitigate these issues, SEO features are now disabled by default.
+
+If you were affected by one of the issues above, we recommend that you read the [Improve performances](https://nuxt-community.github.io/nuxt-i18n/seo.html#improving-performance) section to enable SEO only where you need it.
+
+If you'd like to restore the old behaviour, you can reenable SEO features globally by setting the `seo` option to `true`:
+
+```js
+{
+  seo: true
+}
+```
+
+### preserveState can't be set anymore
+
+It was previously possible to manually set `preserveState` on **nuxt-i18n**'s store module, which would actually result in unexpected behaviours when using server-side rendering. This option has been removed altogether and the module's `preserveState` option is now [set automatically](https://github.com/nuxt-community/nuxt-i18n/blob/05e9d1f80715cc23a545adf4303e49af3ee40ac3/src/plugins/main.js#L77).
+
+If you were using the `preserveState` configuration option before, it can be safely removed:
+
+```patch
+ {
+   vuex: {
+-    preserveState: true,
+     // other configuration options
+   }
+ }
+```
+
+### Store module options have been flattened and renamed
+
+The `vuex` configuration option used to expose a `mutations` property where each mutation could be disabled or renamed. For the sake of simplicity, it isn't possible to rename these mutations anymore, the `mutations` property has been dropped to flatten the configuration and each option has been renamed to better reflect what it does.
+
+```patch
+ {
+   vuex: {
+-    mutations: {
+-      setLocale: 'SET_LOCALE_MUTATION',
+-      setMessages: 'SET_MESSAGE_MUTATION',
+-      setRouteParams: 'SET_ROUTE_PARAMS_MUTATION'
+-    }
++    syncLocale: true,
++    syncMessages: true,
++    syncRouteParams: true
+   },
+ }
+ ```
+
+## Upgrading from 4.x to 5.x
+
+Please refer to [**vue-i18n**'s changelog](https://github.com/kazupon/vue-i18n/blob/dev/CHANGELOG.md#800-2018-06-23) for more information on breaking changes in **nuxt-i18n 5.x**.
+
 ## Upgrading from 3.x to 4.x
 
 ### In-component options key