Add missing docs to upgrading guide for cache config

docs/en/04_Changelogs/4.0.0.md

@@ -1599,6 +1599,7 @@ and have a slightly different API (e.g. `set()` instead of `save()`).
 
 Before:
 
+
     :::php
     $cache = Cache::factory('myCache');
      
@@ -1619,6 +1620,7 @@ Before:
 
 After:
 
+
     :::php
     use Psr\SimpleCache\CacheInterface;
     $cache = Injector::inst()->get(CacheInterface::class . '.myCache');
@@ -1636,6 +1638,21 @@ After:
     
     $cache->delete('myCacheKey');
 
+
+With the necessary minimal config in `_config/mycache.yml`
+
+
+    :::yml
+    ---
+    Name: mycache
+    ---
+    SilverStripe\Core\Injector\Injector:
+      Psr\SimpleCache\CacheInterface.myCache:
+        factory: SilverStripe\Core\Cache\CacheFactory
+        constructor:
+          namespace: 'mycache'
+
+
 #### Configuration Changes
 
 Caches are now configured through dependency injection services instead of PHP.