Add comment to Flipper patch on when we can remove it

A version of this change was accepted upstream, so once we're using a
version of Flipper that includes it, we can remove this file and simply
pass an additional option to the cache store's constructor.

lib/feature/active_support_cache_store_adapter.rb

@@ -4,6 +4,10 @@
 # This class was already nested this way before moving to a separate file
 class Feature
   class ActiveSupportCacheStoreAdapter < Flipper::Adapters::ActiveSupportCacheStore
+    # This patch represents https://github.com/jnunemaker/flipper/pull/512. In
+    # Flipper 0.21.0 and later, we can remove this and just pass `write_through:
+    # true` to the constructor in `Feature.build_flipper_instance`.
+
     extend ::Gitlab::Utils::Override
 
     override :enable