Add comment for RVM bin added to /Users/xiaogwu/.rvm/gems/ruby-1.9.3-p429/bin:/Users/xiaogwu/.rvm/gems/ruby-1.9.3-p429@global/bin:/Users/xiaogwu/.rvm/rubies/ruby-1.9.3-p429/bin:/Users/xiaogwu/.rvm/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:~/bin