Adding Fragment Cache to AMS

It's an upgrade based on the new Cache implementation #693.
It allows to use the Rails conventions to cache
specific attributes or associations.
It's based on the Cache Composition implementation.

README.md

@@ -271,7 +271,10 @@ The options are the same options of ```ActiveSupport::Cache::Store```, plus
 a ```key``` option that will be the prefix of the object cache
 on a pattern ```"#{key}/#{object.id}-#{object.updated_at}"```.
 
+The cache support is optimized to use the cached object in multiple request. An object cached on an ```show``` request will be reused at the ```index```. If there is a relationship with another cached serializer it will also be created and reused automatically.
+
 **[NOTE] Every object is individually cached.**
+
 **[NOTE] The cache is automatically expired after update an object but it's not deleted.**
 
 ```ruby
@@ -295,6 +298,27 @@ On this example every ```Post``` object will be cached with
 the key ```"post/#{post.id}-#{post.updated_at}"```. You can use this key to expire it as you want,
 but in this case it will be automatically expired after 3 hours.
 
+### Fragmenting Caching
+
+If there is some API endpoint that shouldn't be fully cached, you can still optmise it, using Fragment Cache on the attributes and relationships that you want to cache.
+
+You can define the attribute by using ```only``` or ```except``` option on cache method.
+
+**[NOTE] Cache serializers will be used at their relationships**
+
+Example:
+
+```ruby
+class PostSerializer < ActiveModel::Serializer
+  cache key: 'post', expires_in: 3.hours, only: [:title]
+  attributes :title, :body
+
+  has_many :comments
+
+  url :post
+end
+```
+
 ## Getting Help
 
 If you find a bug, please report an [Issue](https://github.com/rails-api/active_model_serializers/issues/new).