def add_conditional_formatting(cells, rules)
      cf = ConditionalFormatting.new( :sqref => cells )
      cf.add_rules rules
      conditional_formattings << cf
      conditional_formattings
    end
Add conditional formatting to this worksheet.

@param [String] cells The range to apply the formatting to
@param [Array|Hash] rules An array of hashes (or just one) to create Conditional formatting rules from.
@example This would format column A whenever it is FALSE.
# for a longer example, see examples/example_conditional_formatting.rb (link below)
worksheet.add_conditional_formatting( "A1:A1048576", { :type => :cellIs, :operator => :equal, :formula => "FALSE", :dxfId => 1, :priority => 1 }

@see ConditionalFormattingRule#initialize
@see file:examples/example_conditional_formatting.rb