add find_template doc

book/Views.markdown

@@ -487,3 +487,45 @@ Or, specify an explicit Hash of local variables:
 
 This is typically used when rendering templates as partials from within
 other templates.
+
+Looking Up Template Files
+-------------------------
+
+The `find_template` helper is used to find template files for rendering:
+
+    find_template settings.views, 'foo', Tilt[:haml] do |file|
+      puts "could be #{file}"
+    end
+
+This is not really useful. But it is useful that you can actually override this
+method to hook in your own lookup mechanism. For instance, if you want to be
+able to use more than one view directory:
+
+    set :views, ['views', 'templates']
+
+    helpers do
+      def find_template(views, name, engine, &block)
+        Array(views).each { |v| super(v, name, engine, &block) }
+      end
+    end
+
+Another example would be using different directories for different engines:
+
+    set :views, :sass => 'views/sass', :haml => 'templates', :default => 'views'
+
+    helpers do
+      def find_template(views, name, engine, &block)
+        _, folder = views.detect { |k,v| engine == Tilt[k] }
+        folder ||= views[:default]
+        super(folder, name, engine, &block)
+      end
+    end
+
+You can also easily wrap this up in an extension and share with others!
+
+Note that `find_template` does not check if the file really exists but
+rather calls the given block for all possible paths. This is not a performance
+issue, since +render+ will use +break+ as soon as a file is found. Also,
+template locations (and content) will be cached if you are not running in
+development mode. You should keep that in mind if you write a really crazy
+method.