Add a comment about JS runtimes to Gemfile

As mentioned in #3078 there may be some confusion about the asset
pipeline's dependency on an ExecJS compatible javascript runtime.

On OSX it should just work, Linux needs Node.js (recommended) or
TheRubyRacer.

This adds a comment to the :assets group to give people more
information.

Gemfile

@@ -109,6 +109,14 @@ gem 'client_side_validations'
 
 group :assets do
   gem 'sass-rails', '3.1.4'
+
+  # Windows and OSX have an execjs compatible runtime built-in, Linux users should
+  # install Node.js or use 'therubyracer'.
+  #
+  # See https://github.com/sstephenson/execjs#readme for more supported runtimes
+  
+  # gem 'therubyracer', :platform => :ruby
+  
   gem 'handlebars_assets'
   gem 'uglifier'
 end