Update README

Author: Lee Richmond

README.md

@@ -8,6 +8,35 @@
 
 Supported Rails versions: >= 4.1
 
+### Upgrading from 0.10.x to 0.11.x
+
+Due to a backwards-incompatibility introduced in the underlying
+[jsonapi-rb](http://jsonapi-rb.org) gem, specifying custom serializers
+now works slightly differently.
+
+Before:
+
+```ruby
+# app/serializers/serializable_post.rb
+
+has_many :comments, class: SerializableSpecialComment
+```
+
+and/or
+
+```ruby
+render_jsonapi(post, class: SerializableSpecialPost)
+```
+
+This is now all handled at the controller level:
+
+```ruby
+render_jsonapi(post, class: {
+  Post: SerializableSpecialPost,
+  Comment: SerializableSpecialComment
+})
+```
+
 ### Upgrading to 0.10
 
 `sideload_whitelist` has been moved from the resource to the controller: