Add Conditional Class Selectors in Haml as a rails til.

rails/conditional-class-selectors-in-haml.md

@@ -0,0 +1,21 @@
+# Conditional Class Selectors in Haml
+
+You can assign a class selector to a tag in HAML like so:
+
+```ruby
+%div.active
+```
+
+You can conditionally assign a class selector in a concise manner like so:
+
+```ruby
+%div{ class: ( "active" if @thing.active? ) }
+```
+
+You can do multiple conditional class selectors with array syntax:
+
+```ruby
+%div{ class: [ ("active" if @thing.active?), ("highlight" if @thing.important?) ] }
+```
+
+[source](http://stackoverflow.com/questions/3453560/append-class-if-condition-is-true-in-haml-with-rails)