Adding documentation on conditional attributes

Adding documentation and short example ([from this pull request](rails-api#1403)) on conditional attributes.

Author: lambda2

docs/general/serializers.md

@@ -34,6 +34,18 @@ Serialization of the resource `title`
 | `attribute :title { 'A Different Title'}` | `{ title: 'A Different Title' } `
 | `attribute :title`<br>`def title 'A Different Title' end` | `{ title: 'A Different Title' }`
 
+An `if` or `unless` parameter can make an attribute conditional. It take a symbol of a method name on the serializer.
+
+e.g.
+
+```ruby
+attribute :private_data, if: :is_current_user?
+
+def is_current_user?
+  object.id == current_user.id
+end
+```
+
 [PR please for conditional attributes:)](https://github.com/rails-api/active_model_serializers/pull/1403)
 
 ### Associations