Add upgrade notes that include 2.0.10 instructions [skip ci]

Author: eridiumdev

UPGRADE.md

@@ -0,0 +1,34 @@
+# Upgrading Instructions for Yii 2.0 Redis Extension
+
+This file contains the upgrade notes for Yii 2.0 Redis Extension. These notes highlight changes that
+could break your application when you upgrade extension from one version to another.
+
+Upgrading in general is as simple as updating your dependency in your composer.json and
+running `composer update`. In a big application however there may be more things to consider,
+which are explained in the following.
+
+> Note: The following upgrading instructions are cumulative. That is,
+if you want to upgrade from version A to version C and there is
+version B between A and C, you need to follow the instructions
+for both A and B.
+
+Upgrade from 2.0.10
+------------------
+yii2-redis now provides a better support for Redis in cluster mode.
+
+If you're using redis in cluster mode and want to use `MGET` and `MSET` effectively, you will need to supply a
+[hash tag](https://redis.io/topics/cluster-spec#keys-hash-tags) to allocate cache keys to the same hash slot.
+```
+\Yii::$app->cache->multiSet([
+    'posts{user1}' => 123,
+    'settings{user1}' => [
+        'showNickname' => false,
+        'sortBy' => 'created_at',
+    ],
+    'unreadMessages{user1}' => 5,
+]);
+```
+
+You might also want to force cluster mode for every request by setting `'forceClusterMode' => true` in your Redis component config. Otherwise an implicit check that executes Redis command `'CLUSTER INFO'` will be made on each request.
+
+If you do not intend on using Redis in cluster mode, it is advisable to set `'forceClusterMode' => false` to avoid additional overhead and possible troubles when extending `yii\redis\Connection::executeCommand`, because executing `'CLUSTER INFO'` will return an error on a single-node Redis.