update readme

Author: shishirmk

README.md

@@ -174,7 +174,7 @@ By default, attributes are read directly from the model property of the same nam
 ```ruby
 class MovieSerializer
   include FastJsonapi::ObjectSerializer
-  
+
   attribute :name
 end
 ```
@@ -184,9 +184,9 @@ Custom attributes that must be serialized but do not exist on the model can be d
 ```ruby
 class MovieSerializer
   include FastJsonapi::ObjectSerializer
-  
+
   attributes :name, :year
-  
+
   attribute :name_with_year do |object|
     "#{object.name} (#{object.year})"
   end
@@ -198,7 +198,7 @@ The block syntax can also be used to override the property on the object:
 ```ruby
 class MovieSerializer
   include FastJsonapi::ObjectSerializer
-  
+
   attribute :name do |object|
     "#{object.name} Part 2"
   end
@@ -254,7 +254,7 @@ related to a current authenticated user. The `options[:params]` value covers the
 cases by allowing you to pass in a hash of additional parameters necessary for
 your use case.
 
-Leveraging the new params is easy, when you define a custom attribute with a
+Leveraging the new params is easy, when you define a custom attribute or relationship with a
 block you opt-in to using params by adding it as a block parameter.
 
 ```ruby
@@ -267,6 +267,11 @@ class MovieSerializer
     # in here, params is a hash containing the `:current_user` key
     params[:current_user].is_employee? ? true : false
   end
+
+  belongs_to :primary_agent do |movie, params|
+    # in here, params is a hash containing the `:current_user` key
+    params[:current_user].is_employee? ? true : false
+  end
 end
 
 # ...
@@ -275,7 +280,7 @@ serializer = MovieSerializer.new(movie, {params: {current_user: current_user}})
 serializer.serializable_hash
 ```
 
-Custom attributes that only receive the resource are still possible by defining
+Custom attributes and relationships that only receive the resource are still possible by defining
 the block to only receive one argument.
 
 ### Customizable Options