Add ruby highlight

Readme.md

@@ -3,31 +3,39 @@
 A simple date validator for Rails 3. Compatible with Ruby 1.8.7, 1.9.2 and
 Rubinius 1.2.2.
 
+```ruby
     $ gem sources -a http://gemcutter.org/
     $ gem install date_validator
+```
 
 And I mean simple. In your model:
 
+```ruby
     validates :expiration_date,
               :date => {:after => Proc.new { Time.now },
                         :before => Proc.new { Time.now + 1.year } }
     # Using Proc.new prevents production cache issues
+```
 
 If you want to check the date against another attribute, you can pass it
 a Symbol instead of a block:
 
+```ruby
     # Ensure the expiration date is after the packaging date
     validates :expiration_date,
               :date => {:after => :packaging_date}
+```
 
 For now the available options you can use are `:after`, `:before`,
 `:after_or_equal_to` and `:before_or_equal_to`.
 
 If you want to specify a custom message, you can do so in the options hash:
 
+```ruby
     validates :start_date,
       :date => {:after => Date.today, :message => 'must be after today'},
       :on => :create
+```
 
 Pretty much self-explanatory! :)