Append row to csv file Ruby 1.9 CSV lib

Using Ruby 1.9 and CSV lib, I can't seem to append a row. The example in the documentation opens the file, and overwrites the row. What is the correct way to append rows to the document?

Example from documentation:

require 'csv'
CSV.open("path/to/file.csv", "wb") do |csv|
  csv << ["row", "of", "CSV", "data"]
  csv << ["another", "row"]
  # ...
end