Added conditional validation example

gems/reform/api.md

@@ -210,6 +210,8 @@ The model won't be touched, its values are still the original ones.
 
     model.artist.name #=> "Duran Duran"
 
+You can find more options and validation examples [here](/gems/reform/validation.html).
+
 ### Deserialization and Populator
 
 Very often, you need to give Reform some information how to create or find nested objects when `validate`ing. This directive is called _populator_ and [documented here](http://trailblazer.to/gems/reform/populator.html).

gems/reform/validation.md

@@ -89,6 +89,33 @@ At any time you can extend an existing group using `:inherit`.
 
 This appends validations to the existing `:email` group.
 
+## Conditional Validation
+
+Sometimes you need to run validations based on data of your form. This can be easily done with conditional validations.
+
+    require "reform/form/dry"
+
+    class Fund::CreateCallForm < Reform::Form
+      feature Reform::Form::Dry
+
+      property :type
+      property :fund
+
+      validation do
+        required(:type) { filled? & included_in?(TRANSACTION_TYPES['Fund::Call']) }
+      end
+
+      validation if: -> (results) { rebalance? } do
+        required(:fund).filled
+      end
+
+      def rebalance?
+        self.type == 'rebalance'
+      end
+    end
+
+Our form has two properties `type` and `fund` and fund is required only if type is `type == 'rebalance'`.
+
 ## Dry-validation
 
 Dry-validation is the preferred backend for defining and executing validations.