Adding documentation on conditional attributes

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

Adding lambda literal notation and example.

Adding lambda literal notation and example, and fixing typo.

Removing PR reminder

Adding Changelog entry

Moving CHANGELOG entry under master (unreleased)

Use option instead of parameter

Author: lambda2

CHANGELOG.md

@@ -9,6 +9,7 @@ Features:
 Fixes:
 
 Misc:
+- [#1734](https://github.com/rails-api/active_model_serializers/pull/1734) Adds documentation for conditional attribute (@lambda2)
 
 ### [v0.10.0 (2016-05-17)](https://github.com/rails-api/active_model_serializers/compare/4a2d9853ba7...v0.10.0)
 

docs/general/serializers.md

@@ -34,7 +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' }`
 
-[PR please for conditional attributes:)](https://github.com/rails-api/active_model_serializers/pull/1403)
+An `if` or `unless` option can make an attribute conditional. It takes a symbol of a method name on the serializer, or a lambda literal.
+
+e.g.
+
+```ruby
+attribute :private_data, if: :is_current_user?
+attribute :another_private_data, if: -> { scope.admin? }
+
+def is_current_user?
+  object.id == current_user.id
+end
+```
 
 ### Associations