add documentation for node_js_installed?

Author: hone

lib/language_pack/ruby.rb

@@ -692,6 +692,8 @@ def node_bp_bin_path
     "#{Dir.pwd}/#{NODE_BP_PATH}"
   end
 
+  # checks if node.js is installed via the official heroku-buildpack-nodejs using multibuildpack
+  # @return [Boolean] true if it's detected and false if it isn't
   def node_js_installed?
     @node_js_installed ||= run("#{node_bp_bin_path}/node -v") && $?.success?
   end