Adds caching

app/serializers/activity_serializer.rb

 class ActivitySerializer
   include JSONAPI::Serializer
   attributes :name, :author_id, :description, :client
+
+  cache_options store: Rails.cache, namespace: 'jsonapi-serializer', expires_in: 1.hour
 end

app/serializers/user_serializer.rb

   include JSONAPI::Serializer
   attributes :email, :username
   has_many :activities
+
+  cache_options store: Rails.cache, namespace: 'jsonapi-serializer', expires_in: 1.hour
 end