Add README instructions for using sparse fieldsets

Author: Erol

README.md

@@ -32,6 +32,7 @@ Fast JSON API serialized 250 records in 3.01 ms
   * [Params](#params)
   * [Conditional Attributes](#conditional-attributes)
   * [Conditional Relationships](#conditional-relationships)
+  * [Sparse Fieldsets](#sparse-fieldsets)
 * [Contributing](#contributing)
 
 
@@ -388,6 +389,21 @@ serializer = MovieSerializer.new(movie, { params: { admin: current_user.admin? }
 serializer.serializable_hash
 ```
 
+### Sparse Fieldsets
+
+Attributes and relationships can be selectively returned per record type by using the `fields` option.
+
+```ruby
+class MovieSerializer
+  include FastJsonapi::ObjectSerializer
+
+  attributes :name, :year
+end
+
+serializer = MovieSerializer.new(movie, { fields: { movie: [:name] } })
+serializer.serializable_hash
+```
+
 ### Customizable Options
 
 Option | Purpose | Example